Final Air Brake Check | High Road Online CDL Training (2022)

STEP 7: Final Air Brake Check

Do the following checks instead of the hydraulic brake check described in Section 2: Step 7 - Check Brake System.

Test low pressure warning signal:

Final Air Brake Check | High Road Online CDL Training (1)

Shut the engine off when you have enough air pressure that the low-pressure warning signal is not on. Turn the electrical power on, and step on and off the brake pedal to reduce air tank pressure. The low air pressure warning signal must come on before the pressure drops to less than 60 psi in the air tank, or the tank with the lowest air pressure in dual air systems.

If the warning signal does not work, you could lose air pressure and not know it. This could cause sudden emergency braking in a single circuit air system. In dual systems the stopping distance will be increased. Only limited braking can be done before the spring brakes come on.

Check that spring brakes come on automatically:

Chock the wheels, release the parking brakes when you have enough air pressure to do it and shut the engine off. Step on and off the brake pedal to reduce the air tank pressure. The “parking brake” knob should pop out when the air pressure falls to the manufacturerʼs specification, usually in a range between 20 and 45 psi. This causes the spring brakes to come on.

Check rate of air pressure buildup:

When the engine is at operating RPM, the pressure should build from 85 to 100 psi within 45 seconds in dual air systems (if the vehicle has larger than minimum air tanks, the buildup time can be longer and still be safe. Check the manufacturerʼs specifications). In single air systems (pre-1975), typical requirements are pressure build-up from 50 to 90 psi within 3 minutes with the engine at an idle speed of 600-900 RPM.

If air pressure does not build up fast enough, the pressure may drop too low during driving, requiring an emergency stop. Do not drive until you get the problem repaired.

Test air leakage rate:

With a fully-charged air system (typically 125 psi), turn off the engine, release the parking brake, and time the air pressure drop. The loss rate should be less than 2 psi in 1 minute for single vehicles and less than 3 psi in 1 minute for combination vehicles. Then apply 90 psi or more with the brake pedal. After the initial pressure drop, if the air pressure falls more than 3 psi in 1 minute for single vehicles and more than 4 psi for combination vehicles, the air loss rate is too much. Check for air leaks, and repair before driving the vehicle. Otherwise, you could lose your brakes while driving.

Check air compressor governor cut-in and cut-out pressures:

Final Air Brake Check | High Road Online CDL Training (2)

Pumping by the air compressor should start at about 100 psi and stop at about 125 psi (check manufacturerʼs specifications). Run the engine at a fast idle. The air governor should cut out the air compressor at about the manufacturerʼs specified pressure. The air pressure shown by your gauge(s) will stop rising. With the engine idling, step on and off the brake to reduce the air tank pressure. The compressor should cut in at about the manufacturerʼs specified cut-in pressure. The pressure should begin to rise.

If the air governor does not work as described above, it may need to be repaired. A governor that does not work right may not keep enough air pressure for safe driving.

Test parking brake:

Stop the vehicle, put the parking brake on, and gently pull against it in a low gear to test that the parking brake will hold.

Test service brakes:

Wait for normal air pressure, release the parking brake, move the vehicle forward slowly (about 5 mph) and apply the brakes firmly using the brake pedal. Note any vehicle “pulling” to one side, unusual feel or delayed stopping action.

This test may show you problems that you otherwise would not know about until you needed the brakes on the road.

Test Your Knowledge

  • What is a dual air brake system?
  • What are slack adjusters?
  • How can you check slack adjusters?
  • How can you test the low pressure warning signal?
  • How can you check that the spring brakes come on automatically?
  • What are the maximum leakage rates?

Study sections 5.2 and 5.3 if you can't answer all of these questions.

This may be getting repetitive for you. But this number has come up many times now so you know it's very important to remember. Once again, the low air pressure warning device should activate before 60 psi! As you can imagine from the number of times this has shown up, this will very likely be asked on your written exam.

Here's another figure we've seen a few times now. You must memorize that the spring brakes will activate when air pressure drops to between 20 and 45 psi.

Side note: Do you remember when the low air pressure warning device must activate? If you said before air pressure reaches 60 psi, you're correct!

This must be memorized and it is asked frequently on the written exam. Air pressure should build from 85 to 100 psi within 45 seconds in a dual air system.

It is very unlikely that you'll be asked about vehicles built before 1975 (as discussed in the remainder of this paragraph).

This is extremely important and is asked about on the written exam very frequently! Memorize the following:

  • With all brakes released, the air loss rate should be less than 2 psi in 1 minute for single vehicles.
  • With all brakes released, the air loss rate should be less than 3 psi in 1 minute for combination vehicles.

Once again, this is very important and is almost always asked on the written exam. Remember:

  • With the service brake depressed, the air loss rate should be less than 3 psi in 1 minute for single vehicles.
  • With the service brake depressed, the air loss rate should be less than 4 psi in 1 minute for combination vehicles.

We've seen these numbers before and it's important that you know them. Here's what to memorize:

  • Cut-In Pressure: About 100 psi. This is when the air governor tells the air compressor to begin pumping air into the air tanks.
  • Cut-Out Pressure: About 125 psi. This is when the air governor tells the air compressor to stop pumping air into the air tanks.

Combination Vehicle:

A vehicle with two separate parts - the power unit (tractor) and the trailer. Tractor-trailers are considered combination vehicles.

OWI:

Operating While Intoxicated

Review Questions - Click On The Picture To Begin...

Final Air Brake Check | High Road Online CDL Training (3)

With a fully charged air system and the service brake released, what is the maximum allowable air loss rate?

  • Less than 2 psi in 1 minute for single vehicles and less than 3 psi in 1 minute for combination vehicles
  • Less than 3 psi in 1 minute for single vehicles and more than 4 psi for combination vehicles
  • Less than 4 psi in 1 minute for single vehicles and more than 3 psi for combination vehicles
  • Less than 3 psi in 1 minute for single vehicles and less than 2 psi in 1 minute for combination vehicles

Quote From The CDL Manual:

Test air leakage rate: With a fully-charged air system (typically 125 psi), turn off the engine, release the service brake, and time the air pressure drop. The loss rate should be less than 2 psi in 1 minute for single vehicles and less than 3 psi in 1 minute for combination vehicles. Then apply 90 psi or more with the brake pedal. After the initial pressure drop, if the air pressure falls more than 3 psi in 1 minute for single vehicles and more than 4 psi for combination vehicles, the air loss rate is too much. Check for air leaks, and repair before driving the vehicle. Otherwise, you could lose your brakes while driving.

TruckingTruth's Advice:

You really need to memorize the allowable air leakage rates. This will very likely come up on your written exam and will come up again during the pre-trip exam. Here's what you should memorize (create flash cards if you have to).

With the service brakes released (not depressing the brake pedal):

  • Air loss rate should be less than 2 psi in 1 minute for a single vehicle.
  • Air loss rate should be less than 3 psi in 1 minute for combination vehicles (vehicles with a trailer).

With the service brakes depressed (pressing the brake pedal):

  • Air loss rate should be less than 3 psi in 1 minute for a single vehicle.
  • Air loss rate should be less than 4 psi in 1 minute for combination vehicles (vehicles with a trailer).

Be sure to have that memorized. Very important!!

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(Video) inside cabin and air brake test ( Russian and English )

Which of these statements accurately defines the maximum amount of air loss allowed with the service brake released?

  • The loss rate should be less than 1 psi in 1 minute for single vehicles and less than 2 psi in 1 minute for combination vehicles
  • None of these answers are correct
  • The loss rate should be less than 3 psi in 1 minute for single vehicles and less than 4 psi in 1 minute for combination vehicles
  • The loss rate should be less than 3 psi in 2 minutes for single vehicles and less than 4 psi in 2 minutes for combination vehicles

Quote From The CDL Manual:

Test air leakage rate: With a fully-charged air system (typically 125 psi), turn off the engine, release the service brake, and time the air pressure drop. The loss rate should be less than 2 psi in 1 minute for single vehicles and less than 3 psi in 1 minute for combination vehicles. Then apply 90 psi or more with the brake pedal. After the initial pressure drop, if the air pressure falls more than 3 psi in 1 minute for single vehicles and more than 4 psi for combination vehicles, the air loss rate is too much. Check for air leaks, and repair before driving the vehicle. Otherwise, you could lose your brakes while driving.

TruckingTruth's Advice:

You really need to memorize the allowable air leakage rates. This will very likely come up on your written exam and will come up again during the pre-trip exam. Here's what you should memorize (create flash cards if you have to).

With the service brakes released (not depressing the brake pedal):

  • Air loss rate should be less than 2 psi in 1 minute for a single vehicle.
  • Air loss rate should be less than 3 psi in 1 minute for combination vehicles (vehicles with a trailer).

With the service brakes depressed (pressing the brake pedal):

  • Air loss rate should be less than 3 psi in 1 minute for a single vehicle.
  • Air loss rate should be less than 4 psi in 1 minute for combination vehicles (vehicles with a trailer).

Be sure to have that memorized. Very important!!

Prev

Next

What is "Cut-In Air Pressure"?

  • When the governor tells the emergency brakes to engage
  • When the governor tells the air compressor to stop pumping air into the air tanks
  • When the governor tells the service brakes to activate
  • When the governor tells the air compressor to start pumping air into the air tanks

Quote From The CDL Manual:

Check air compressor governor cut-in and cut-out pressures: Pumping by the air compressor should start at about 100 psi and stop at about 125 psi. (Check manufacturer's specifications.) Run the engine at a fast idle. The air governor should cut out the air compressor at about the manufacturer's specified pressure. The air pressure shown by your gauge(s) will stop rising. With the engine idling, step on and off the brake to reduce the air tank pressure. The compressor should cut in at about the manufacturer's specified cut-in pressure. The pressure should begin to rise.

TruckingTruth's Advice:

It is important to memorize the "cut in" and "cut out" levels for the air governor as well as the terminology.

Cut in: When the governor tells the air compressor to start pumping air into the air tanks. This should occur when air pressure falls to about 100 psi.

Cut out: When the governor tells the air compressor to stop pumping air into the air tanks. This should occur when the air pressure has risen to about 125 psi.

Prev

Next

What is the proper way to test the service brakes?

  • Chock the wheels, turn off the engine, then release all brakes to perform a manual inspection of the service brakes
  • Service brakes should only be tested by a certified mechanic
  • Bring the vehicle to highway speed (about 55 mph), check for traffic behind you, and if no traffic is present depress the brakes firmly for about 1 second making note of any unusual stopping action
  • Move the vehicle forward slowly (about 5 mph) and apply the brakes firmly using the brake pedal, making note of any unusual stopping action

Quote From The CDL Manual:

Test service brakes: Wait for normal air pressure, release the parking brake, move the vehicle forward slowly (about 5 mph) and apply the brakes firmly using the brake pedal. Note any vehicle "pulling" to one side, unusual feel or delayed stopping action.

This test may show you problems that you otherwise would not know about until you needed the brakes on the road.

TruckingTruth's Advice:

You will be expected to perform a service brake check during your pre-trip examination.

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Next

At what psi will the air compressor governor tell the air compressor to "cut out"?

  • About 115 psi
  • About 110 psi
  • About 125 psi
  • About 90 psi

Quote From The CDL Manual:

Check air compressor governor cut-in and cut-out pressures: Pumping by the air compressor should start at about 100 psi and stop at about 125 psi. (Check manufacturer's specifications.) Run the engine at a fast idle. The air governor should cut out the air compressor at about the manufacturer's specified pressure. The air pressure shown by your gauge(s) will stop rising. With the engine idling, step on and off the brake to reduce the air tank pressure. The compressor should cut in at about the manufacturer's specified cut-in pressure. The pressure should begin to rise.

TruckingTruth's Advice:

It is important to memorize the "cut in" and "cut out" levels for the air governor as well as the terminology.

Cut in: When the governor tells the air compressor to start pumping air into the air tanks. This should occur when air pressure falls to about 100 psi.

Cut out: When the governor tells the air compressor to stop pumping air into the air tanks. This should occur when the air pressure has risen to about 125 psi.

Prev

Next

At about what psi should the air compressor stop pumping air into the air tanks?

(Video) L.A.B Test - Air Brake Check - Pre-Trip Inspection - CDL Class A

  • 100 psi
  • 125 psi
  • 115 psi
  • 110 psi

Quote From The CDL Manual:

Check air compressor governor cut-in and cut-out pressures: Pumping by the air compressor should start at about 100 psi and stop at about 125 psi. (Check manufacturer's specifications.) Run the engine at a fast idle. The air governor should cut out the air compressor at about the manufacturer's specified pressure. The air pressure shown by your gauge(s) will stop rising. With the engine idling, step on and off the brake to reduce the air tank pressure. The compressor should cut in at about the manufacturer's specified cut-in pressure. The pressure should begin to rise.

Prev

Next

At proper engine operating RPM, how quickly should air pressure build from 85 to 100 psi with a dual air system?

  • Within 1 minute
  • Within 30 seconds
  • Within 45 seconds
  • Within 25 seconds

Quote From The CDL Manual:

Check rate of air pressure buildup: When the engine is at operating RPM, the pressure should build from 85 to 100 psi within 45 seconds in dual air systems. (If the vehicle has larger than minimum air tanks, the buildup time can be longer and still be safe. Check the manufacturer's specifications.) In single air systems pre-1975), typical requirements are pressure build-up from 50 to 90 psi within 3 minutes with the engine at an idle speed of 600-900 RPM.

Prev

Next

What is the maximum allowable air loss rate for a single vehicle while the service brakes are released?

  • Less than 2 psi in 1 minute
  • Less than 4 psi in 1 minutes
  • Less than 1 psi in 1 minute
  • Less than 3 psi in 1 minute

Quote From The CDL Manual:

Test air leakage rate: With a fully-charged air system (typically 125 psi), turn off the engine, release the service brake, and time the air pressure drop. The loss rate should be less than 2 psi in 1 minute for single vehicles and less than 3 psi in 1 minute for combination vehicles. Then apply 90 psi or more with the brake pedal. After the initial pressure drop, if the air pressure falls more than 3 psi in 1 minute for single vehicles and more than 4 psi for combination vehicles, the air loss rate is too much. Check for air leaks, and repair before driving the vehicle. Otherwise, you could lose your brakes while driving.

TruckingTruth's Advice:

You really need to memorize the allowable air leakage rates. This will very likely come up on your written exam and will come up again during the pre-trip exam. Here's what you should memorize (create flash cards if you have to).

With the service brakes released (not depressing the brake pedal):

  • Air loss rate should be less than 2 psi in 1 minute for a single vehicle.
  • Air loss rate should be less than 3 psi in 1 minute for combination vehicles (vehicles with a trailer).

With the service brakes depressed (pressing the brake pedal):

  • Air loss rate should be less than 3 psi in 1 minute for a single vehicle.
  • Air loss rate should be less than 4 psi in 1 minute for combination vehicles (vehicles with a trailer).

Be sure to have that memorized. Very important!!

Prev

Next

At what psi will the air compressor governor tell the air compressor to "cut-in"?

  • 120 psi
  • 100 psi
  • 110 psi
  • 90 psi

Quote From The CDL Manual:

Check air compressor governor cut-in and cut-out pressures: Pumping by the air compressor should start at about 100 psi and stop at about 125 psi. (Check manufacturer's specifications.) Run the engine at a fast idle. The air governor should cut out the air compressor at about the manufacturer's specified pressure. The air pressure shown by your gauge(s) will stop rising. With the engine idling, step on and off the brake to reduce the air tank pressure. The compressor should cut in at about the manufacturer's specified cut-in pressure. The pressure should begin to rise.

TruckingTruth's Advice:

It is important to memorize the "cut in" and "cut out" levels for the air governor as well as the terminology.

Cut in: When the governor tells the air compressor to start pumping air into the air tanks. This should occur when air pressure falls to about 100 psi.

Cut out: When the governor tells the air compressor to stop pumping air into the air tanks. This should occur when the air pressure has risen to about 125 psi.

Prev

Next

What is the maximum allowable air loss for a single vehicle with the service brake applied?

  • No more than 5 psi in 1 minute
  • No more than 6 psi in 1 minute
  • No more than 3 psi in 1 minute
  • No more than 4 psi in 1 minute

Quote From The CDL Manual:

Test air leakage rate: With a fully-charged air system (typically 125 psi), turn off the engine, release the service brake, and time the air pressure drop. The loss rate should be less than 2 psi in 1 minute for single vehicles and less than 3 psi in 1 minute for combination vehicles. Then apply 90 psi or more with the brake pedal. After the initial pressure drop, if the air pressure falls more than 3 psi in 1 minute for single vehicles and more than 4 psi for combination vehicles, the air loss rate is too much. Check for air leaks, and repair before driving the vehicle. Otherwise, you could lose your brakes while driving.

TruckingTruth's Advice:

You really need to memorize the allowable air leakage rates. This will very likely come up on your written exam and will come up again during the pre-trip exam. Here's what you should memorize (create flash cards if you have to).

With the service brakes released (not depressing the brake pedal):

  • Air loss rate should be less than 2 psi in 1 minute for a single vehicle.
  • Air loss rate should be less than 3 psi in 1 minute for combination vehicles (vehicles with a trailer).

With the service brakes depressed (pressing the brake pedal):

  • Air loss rate should be less than 3 psi in 1 minute for a single vehicle.
  • Air loss rate should be less than 4 psi in 1 minute for combination vehicles (vehicles with a trailer).

Be sure to have that memorized. Very important!!

Prev

Next

(Video) Mass CDL Air Brake Test 2022 | Class A Training

The air compressor should start pumping air into the air tanks when pressure drops below what psi?

  • 100 psi
  • 125 psi
  • 115 psi
  • 110 psi

Quote From The CDL Manual:

Check air compressor governor cut-in and cut-out pressures: Pumping by the air compressor should start at about 100 psi and stop at about 125 psi. (Check manufacturer's specifications.) Run the engine at a fast idle. The air governor should cut out the air compressor at about the manufacturer's specified pressure. The air pressure shown by your gauge(s) will stop rising. With the engine idling, step on and off the brake to reduce the air tank pressure. The compressor should cut in at about the manufacturer's specified cut-in pressure. The pressure should begin to rise.

Prev

Next

Which of these statements accurately defines the maximum amount of air loss allowed with the service brake engaged?

  • The loss rate should be less than 2 psi in 1 minute for single vehicles and less than 3 psi in 1 minute for combination vehicles
  • The loss rate should be less than 3 psi in 2 minutes for single vehicles and less than 4 psi in 2 minutes for combination vehicles
  • None of these answers are correct
  • The loss rate should be less than 4 psi in 1 minute for single vehicles and less than 5 psi in 1 minute for combination vehicles

Quote From The CDL Manual:

Test air leakage rate: With a fully-charged air system (typically 125 psi), turn off the engine, release the service brake, and time the air pressure drop. The loss rate should be less than 2 psi in 1 minute for single vehicles and less than 3 psi in 1 minute for combination vehicles. Then apply 90 psi or more with the brake pedal. After the initial pressure drop, if the air pressure falls more than 3 psi in 1 minute for single vehicles and more than 4 psi for combination vehicles, the air loss rate is too much. Check for air leaks, and repair before driving the vehicle. Otherwise, you could lose your brakes while driving.

TruckingTruth's Advice:

You really need to memorize the allowable air leakage rates. This will very likely come up on your written exam and will come up again during the pre-trip exam. Here's what you should memorize (create flash cards if you have to).

With the service brakes released (not depressing the brake pedal):

  • Air loss rate should be less than 2 psi in 1 minute for a single vehicle.
  • Air loss rate should be less than 3 psi in 1 minute for combination vehicles (vehicles with a trailer).

With the service brakes depressed (pressing the brake pedal):

  • Air loss rate should be less than 3 psi in 1 minute for a single vehicle.
  • Air loss rate should be less than 4 psi in 1 minute for combination vehicles (vehicles with a trailer).

Be sure to have that memorized. Very important!!

Prev

Next

What is the maximum allowable air loss rate for a combination vehicle when 90 psi or more is applied with the brake pedal?

  • No more than 3 psi in 1 minute
  • No more than 4 psi in 1 minute
  • No more than 2 psi in 1 minute
  • No more than 1 psi in 1 minute

Quote From The CDL Manual:

Test air leakage rate: With a fully-charged air system (typically 125 psi), turn off the engine, release the service brake, and time the air pressure drop. The loss rate should be less than 2 psi in 1 minute for single vehicles and less than 3 psi in 1 minute for combination vehicles. Then apply 90 psi or more with the brake pedal. After the initial pressure drop, if the air pressure falls more than 3 psi in 1 minute for single vehicles and more than 4 psi for combination vehicles, the air loss rate is too much. Check for air leaks, and repair before driving the vehicle. Otherwise, you could lose your brakes while driving.

TruckingTruth's Advice:

You really need to memorize the allowable air leakage rates. This will very likely come up on your written exam and will come up again during the pre-trip exam. Here's what you should memorize (create flash cards if you have to).

With the service brakes released (not depressing the brake pedal):

  • Air loss rate should be less than 2 psi in 1 minute for a single vehicle.
  • Air loss rate should be less than 3 psi in 1 minute for combination vehicles (vehicles with a trailer).

With the service brakes depressed (pressing the brake pedal):

  • Air loss rate should be less than 3 psi in 1 minute for a single vehicle.
  • Air loss rate should be less than 4 psi in 1 minute for combination vehicles (vehicles with a trailer).

Be sure to have that memorized. Very important!!

Prev

Next

At what psi should the low air warning light and buzzer activate?

  • 80 psi
  • 100 psi
  • 120 psi
  • None of these are correct

Quote From The CDL Manual:

Test low pressure warning signal: Shut the engine off when you have enough air pressure that the low-pressure warning signal is not on. Turn the electrical power on, and step on and off the brake pedal to reduce air tank pressure. The low air pressure warning signal must come on before the pressure drops to less than 60 psi in the air tank, or the tank with the lowest air pressure in dual air systems.


If the warning signal does not work, you could lose air pressure and not know it. This could cause sudden emergency braking in a single circuit air system. In dual systems the stopping distance will be increased. Only limited braking can be done before the spring brakes come on.

TruckingTruth's Advice:

It is very important to know how to check the low pressure warning signal. You will learn most of the procedure when studying for the pre-trip exam, but for the written exam, here's what you should know:



  • Build air pressure so the warning light and buzzer are not activated.

  • Turn the engine off.

  • Turn the key to the "on" position (this will allow you to see the warning light and see the buzzer).

  • Begin depressing and releasing the service brake (brake pedal) repeatedly. This will deplete your air pressure and at about 60 psi, the warning light and buzzer should activate.

Prev

Next

How should you properly test the parking brake?

  • Stop the vehicle, put the parking brake on, and gently pull against it in a low gear to test that the parking brake will hold
  • Place chocks against the tires of the truck, release the parking brake, then visually inspect each parking brake
  • Idle the vehicle in the lowest gear possible then apply the parking brake and be sure the vehicle comes to a complete stop
  • Stop the vehicle, put the parking brake on, and pull against it in the highest gear

Quote From The CDL Manual:

Test parking brake: Stop the vehicle, put the parking brake on, and gently pull against it in a low gear to test that the parking brake will hold.

Prev

Next

(Video) Air Brake Practical Test

What is the maximum allowable air loss rate for a combination vehicle with the service brakes released?

  • Less than 2 psi in 1 minute
  • Less than 1 psi in 1 minute
  • There should be no loss of any psi in 1 minute
  • Less than 3 psi in 1 minute

Quote From The CDL Manual:

Test air leakage rate: With a fully-charged air system (typically 125 psi), turn off the engine, release the service brake, and time the air pressure drop. The loss rate should be less than 2 psi in 1 minute for single vehicles and less than 3 psi in 1 minute for combination vehicles. Then apply 90 psi or more with the brake pedal. After the initial pressure drop, if the air pressure falls more than 3 psi in 1 minute for single vehicles and more than 4 psi for combination vehicles, the air loss rate is too much. Check for air leaks, and repair before driving the vehicle. Otherwise, you could lose your brakes while driving.

TruckingTruth's Advice:

You really need to memorize the allowable air leakage rates. This will very likely come up on your written exam and will come up again during the pre-trip exam. Here's what you should memorize (create flash cards if you have to).

With the service brakes released (not depressing the brake pedal):

  • Air loss rate should be less than 2 psi in 1 minute for a single vehicle.
  • Air loss rate should be less than 3 psi in 1 minute for combination vehicles (vehicles with a trailer).

With the service brakes depressed (pressing the brake pedal):

  • Air loss rate should be less than 3 psi in 1 minute for a single vehicle.
  • Air loss rate should be less than 4 psi in 1 minute for combination vehicles (vehicles with a trailer).

Be sure to have that memorized. Very important!!

Prev

Next

If the air governor stops working properly, which of the following could occur?

  • You may get increased condensation in your air tanks
  • The air system may not keep enough air pressure for safe driving
  • All of these answers are correct
  • You could encounter steering difficulties

Quote From The CDL Manual:

Check air compressor governor cut-in and cut-out pressures:

Pumping by the air compressor should start at about 100 psi and stop at about 125 psi (check manufacturer?s specifications). Run the engine at a fast idle. The air governor should cut out the air compressor at about the manufacturer?s specified pressure. The air pressure shown by your gauge(s) will stop rising. With the engine idling, step on and off the brake to reduce the air tank pressure. The compressor should cut in at about the manufacturer?s specified cut-in pressure. The pressure should begin to rise.

If the air governor does not work as described above, it may need to be repaired. A governor that does not work right may not keep enough air pressure for safe driving.

TruckingTruth's Advice:

It is important to memorize the "cut in" and "cut out" levels for the air governor as well as the terminology.

Cut in: When the governor tells the air compressor to start pumping air into the air tanks. This should occur when air pressure falls to about 100 psi.

Cut out: When the governor tells the air compressor to stop pumping air into the air tanks. This should occur when the air pressure has risen to about 125 psi.

Prev

Next

Which of the following steps to checking the low pressure warning signal is incorrect?

  • Step 3: The low air-pressure warning signal must come on before the pressure drops to less than 60 psi in the air tank, or the tank with the lowest air pressure in dual air systems
  • All of these steps are correct
  • Step 1: Shut the engine off when you have enough air pressure that the low-pressure warning signal is not on
  • Step 2: Turn the electrical power on, and step on and off the brake pedal to reduce air tank pressure

Quote From The CDL Manual:

Test low pressure warning signal: Shut the engine off when you have enough air pressure that the low-pressure warning signal is not on. Turn the electrical power on, and step on and off the brake pedal to reduce air tank pressure. The low air pressure warning signal must come on before the pressure drops to less than 60 psi in the air tank, or the tank with the lowest air pressure in dual air systems.

TruckingTruth's Advice:

It is very important to know how to check the low pressure warning signal. You will learn most of the procedure when studying for the pre-trip exam, but for the written exam, here's what you should know:



  • Build air pressure so the warning light and buzzer are not activated.

  • Turn the engine off.

  • Turn the key to the "on" position (this will allow you to see the warning light and see the buzzer).

  • Begin depressing and releasing the service brake (brake pedal) repeatedly. This will deplete your air pressure and at about 60 psi, the warning light and buzzer should activate.

Prev

Next

How can you check that the spring brakes come on automatically?

  • With the engine running and the parking brake on, step on and off the brake pedal to deplete air in the system until the emergency brakes activate
  • With your foot on the service brake, engage and release the parking brake until air is depleted enough for the emergency brake valve to pop out
  • Chock the wheels, turn the engine off, release all brakes, then step on and off the brake pedal to deplete air in the system until the emergency brakes activate
  • Chock the wheels, turn the engine off, release all brakes, then disconnect a brake line and allow the air to deplete which will show if the spring brakes activate automatically

Quote From The CDL Manual:

Check that spring brakes come on automatically: Chock the wheels, release the parking brakes when you have enough air pressure to do it and shut the engine off. Step on and off the brake pedal to reduce the air tank pressure. The "parking brake" knob should pop out when the air pressure falls to the manufacturer's specification, usually in a range between 20 and 40 psi. This causes the spring brakes to come on.

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(Video) Melt Program AZ Air Brake Practical Test Inspection

FAQs

How long should it take for air brakes to build pressure? ›

Air pressure build-up from 85 psi to 100 psi should take 45 seconds or less. While driving, observe the air pressure gauge during build-up to ensure proper air system operation.

What psi do air brakes release? ›

When air tank pressure rises to the "cut-out" level (around 125 pounds per-square-inch or "psi"), the governor stops the compressor from pumping air. When the tank pressure falls to the "cut-in" pressure (around 100 psi), the governor allows the compressor to start pumping again.

What are the maximum leakage rates for an air brake system? ›

What are the maximum leakage rates? Less than four psi in one minute for single vehicles and less than eight psi in one minute for combination vehicles.

How do you do a leak down test on air brakes? ›

Step on the brake and hold the pedal down for one minute. There should be no more than 3 psi of air loss. Listen for any possible leaks.

At what PSI is the safety valve set to open? ›

Explanation The safety valve protects air tanks and the rest of an air brake system from accumulating too much air pressure. The safety valve is usually set to open at 150 psi.

Which air tank drains first? ›

When draining the tanks, the supply tank should always be drained first to prevent accumulated moisture flowing into the next tank being drained. When the supply tank is drained first, any accumulated moisture is removed before it can pass further into the system.

How long should a semi hold air? ›

Quote From The CDL Manual:

Check rate of air pressure buildup: When the engine is at operating RPM, the pressure should build from 85 to 100 psi within 45 seconds in dual air systems. (If the vehicle has larger than minimum air tanks, the buildup time can be longer and still be safe.

Do you push or pull air brakes? ›

Before you can drive a vehicle with air brakes, you must push in the emergency brake button to fill the system with air. As long as the emergency system is pressurized, the emergency brake will remain free. If the system has a leak, the pressure can decrease enough to engage the emergency brake.

What happens if the air pressure gets too low in an air brake system? ›

Explanation Pressing and releasing the brake pedal unnecessarily can let air out faster than the compressor can replace it, reducing the available air pressure. If the air pressure in an air brake system gets too low, the brakes will no longer work.

How much PSI is in a break line? ›

Most of the metal brake lines burst around 15,000 psi. The typical full-lock operating pressures are 900–1,000psi (69 bar) with manual brakes and 1,400-pluspsi ( 96 bar) with power-assisted brakes.

How often should you drain air tanks CDL? ›

Remember: Manual air tank drains must be drained daily in order to get rid of water and oil inside the air tanks. You may be asked about this on the written exam. Make sure you remember that the safety relief valve generally opens at 150 psi.

How much play is allowed in a slack adjuster? ›

Use a tool such as a small pry bar to move the slack adjuster and check the brake's free play – how far the center of the large clevis pin moves before the brake shoes come in contact with the drum. Free play should be between 3/8 and 5/8 of an inch.

What does Cola stand for air brakes? ›

(Governor CUT-IN) the air compressor governor CUT-IN is at no lower than 85-110 psi.

Do you need an air compressor for a leak-down test? ›

You will need an air compressor capable of 100psi. Hook the leak down gauge up to your compressor first and turn the air pressure regulator on the leak down gauge so as the line pressure is 100 psi. Next step is to put the motor on TDC for the cylinder you want to check.

How do you calculate air loss rate? ›

For safety, drivers should test the air-loss rate of the vehicle's brake system. To test the air-loss rate of the brake system, release the spring brakes, establish normal air pressure and shut off the engine. Hold the brake pedal in the fully applied position and observe the air-pressure readings for one minute.

What PSI is not safe to drive on? ›

Answer provided by

The absolute lowest tire pressure you can drive on is 20 psi—and even that isn't recommended. Anything below 20 psi is considered a flat tire, and driving on it can damage your car. If your tires are this low, you should add air. Recommended tire pressures usually range between 32 psi and 35 psi.

Why does a safety valve pop fully open instantly? ›

This increased area under steam pressure makes the pressure much more unbalanced in the direction of the valve discharge opening and therefore pops the valve into a wide open position. When the valve opens with a “pop” the valve seat is preserved from wiredraw caused by slow opening.

Should you fill your tank all the way up? ›

Something else to keep in mind is that topping off your tank could cause gas to spill on the ground, which is a safety and environmental hazard. The bottom line is that topping off your car is dangerous to you and your car, and may cause problems. Your best bet is to stop pumping when the fuel pump clicks off.

What can happen if the brake pedal is pressed and released too often? ›

Explanation On a vehicle with air brakes, pressing and releasing the brake pedal unnecessarily may let air out of the braking system faster than the compressor can replace it. This may cause the system's air pressure to become too low and the brakes to stop working.

Should you drain air compressor after every use? ›

Draining your compressor after every use is not necessary unless you use it very rarely. A good practice would be to drain the compressor tank every couple of days for daily use. This helps you remove the condensate and moisture gathered inside the tank, slowing down the corrosion process.

How fast does wind have to be to flip a semi? ›

It only takes a high-speed wind of less than 60 miles per hour to overturn a tractor trailer. Winds between 40 and 60 mph can pose serious risk, depending on wind direction and truck load weight. Lighter-weight cargo loads are especially vulnerable to overturning.

How long can you idle a semi? ›

Semi-truck fuel tanks can hold between 120 and 150 gallons of fuel, and some semi-trucks have two fuel tanks installed, which means they can hold up to 300 gallons of fuel. This means that a semi-truck can idle for 187,5 hours on a 150-gallon tank of fuel. That is equal to just over 7 and a half days.

How far can a semi get on a full tank? ›

How many miles can a semi truck go on one tank of gas? Semi trucks can go about 2,100 miles on a tank of diesel fuel (not usually gasoline), assuming tanks totaling 300 gallons and an average fuel efficiency of 7 miles per gallon.

Is it better to pump brakes or hold? ›

Essentially, the system pumps the brakes automatically for you, preventing them from locking up. Therefore, according to the experts: You do not have to pump your brakes. In fact, if you pump them while braking hard, you will lose the benefits of the ABS.

What is yellow button in semi? ›

The parking brake control valve — the one with the yellow button — has no Federal Motor Carrier Safety Administration (FMCSA) regulations specifying the pressure at which it must automatically trip to apply the vehicle parking brakes.

What is the most reliable method of checking air brake adjustment? ›

The most reliable method for inspecting brake adjustment is to measure the applied pushrod stroke.
...
Brake adjustment must be inspected regularly
  1. The air lines attach to a square port in the brake chamber body.
  2. A trapezoidal tag is placed under the clamp bolt that indicates the brake chamber's maximum stroke dimension.
28 Nov 2016

At what PSI does the low air pressure warning? ›

A low air warning switch is installed after the supply reservoir to alert you when air pressure drops below a safe level 60 psi (414 kPa). The switch activates either (or a combination of) a buzzer, warning light or a wig-wag (a “flag” that drops into the driver's view).

How can you manually check the slack adjusters? ›

With service brakes in the released position, mark the push rod even with the brake chamber. Make a full brake application and mark the push rod again. Measure between the two marks to determine the length of push-rod travel (stroke).

What should happen if the air pressure in the air brake system tanks falls below 60 psi? ›

The warning light and buzzer should come on before the air pressure drops below 60 psi in either system. If this happens while driving, you should stop right away and safely park the vehicle. If one air system is very low on pressure, either the front or the rear brakes will not be operating fully.

What is maximum straight line breaking? ›

Explanation Maximum straight-line braking is accomplished by simultaneously applying both the front and rear brakes without locking either wheel.

Why are braided brake lines better? ›

Braided stainless steel brake lines can withstand the pressure associated with the brake system and don't swell. As a result, their lifespan is much longer. This means they can maintain high performance over a long period. When brakes are applied, it feels firm and responsive.

What is the best brake pedal ratio? ›

Wilwood recommends a brake pedal ratio of at least 5:1 and for a street car tipping the scales at 2,800-3,500 pounds 6:1 is preferred. See all 24 photos One of the secrets to good brakes is the proper pedal ratio.

Is it OK to leave compressor tank pressurized? ›

With so much air under so much pressure, can you leave an air compressor full? The short answer is yes, it is perfectly safe to leave an air compressor full. However, this assumes that it has been properly maintained, with regular draining to remove accumulated moisture from inside the tank.

How long should it take for a truck to build air? ›

Air pressure build-up from 85 psi to 100 psi should take 45 seconds or less. While driving, observe the air pressure gauge during build-up to ensure proper air system operation. braking, pull over at a safe place and perform the 2 minute / 5 psi leakage test.

How long does it take for a compressor to fill a tank? ›

It will take about 7 minutes.

What is the maximum leakage rate? ›

What are the maximum leakage rates? Less than four psi in one minute for single vehicles and less than eight psi in one minute for combination vehicles.

What causes slack adjusters to fail? ›

“The most common failure of an automatic slack adjuster is when a mechanic or a driver continues to adjust them,” said Kealy. “They are automatic and therefore typically only need to be adjusted when they are installed for the first time or every time a brake job is performed.”

Which way do you turn a slack adjuster? ›

How to Adjust an Automatic Slack Adjuster - YouTube

What is the lag time for air brakes? ›

Brake lag. The time air takes to travel through a properly maintained air brake system, about 4/10 of a second.

Why are my brakes not building pressure? ›

This can be due to a number of problems: a leak in a brake line, a loss of pressure within the master cylinder itself due to a failed seal, or air being introduced into the braking system. Your first reaction to encountering spongy brakes should be to rapidly pump the brake pedal with your foot.

Why does air braking takes more time? ›

Air braking takes more time than hydraulic braking because air: Takes more time to flow through the lines than hydraulic fluid. Some air brakes systems have air alcohol evaporator.

How long does it take to pressure bleed brakes? ›

A mechanic will use a power hoist to lift your car and a power bleeder to bleed the brakes. It would take around 30 minutes. A good mechanic who has done the job several times in the past should be able to get the job done in around 20 minutes.

How often should you drain your air tanks CDL? ›

Explanation To prevent the build up of oil and water in a vehicle's air tanks, manually operated air tank drains should be used at the end of each day of driving.

What is stab braking CDL? ›

Explanation The stab braking method for emergency stops involves fully applying the brakes until they lock up. The brakes are then released until the wheels start rolling, and then the process is repeated.

What does Cola stand for air brakes? ›

(Governor CUT-IN) the air compressor governor CUT-IN is at no lower than 85-110 psi.

Do I need to pump my brake pedal to get pressure? ›

Air in the Lines: The number one most common cause for having to pump your brakes to get them to work is air in the lines. This may be because your brakes were recently serviced but not bled correctly.

What happens if you don't bleed the master cylinder? ›

To put it simply, bleeding the brakes is the process of pushing fluid through a hydraulic brake system to ensure all air bubbles are removed. If brakes aren't bled and air bubbles are trapped within the brake fluid, hydraulic pressure is greatly reduced, making the brakes less efficient.

How can I increase my brake pressure? ›

1) Line pressure can only be increased by either increasing the mechanical pedal ratio or by decreasing the master cylinder diameter. In either case the pedal travel will be increased. 2) Clamping force can only be increased either by increasing the line pressure or by increasing the diameter of the caliper piston(s).

Why do pilots tap the brakes? ›

It's strictly done to stop the wheels from vibrating as they wind down, which can scare the pax and sometimes the pilot at first.

What is a wig wag CDL? ›

Explanation A wig wag is a type of low air pressure warning device in an air brake system. It drops a mechanical arm into the driver's view when the air pressure in the tanks falls below 60 psi.

What is the formula for stopping distance with air brakes? ›

Perception Distance + Reaction Distance + Brake Lag Distance + Braking Distance = Total Stopping Distance.

What is the correct order to bleed brakes? ›

The sequence is as follows: right rear, left rear, right front, left front. See all 10 photos When bleeding brakes, it's important not to force the brake pedal down more than halfway.

Do you have to bleed all 4 when braking? ›

Answer provided by. Yes, many car experts recommend that you bleed all four brakes once you open one of the brake lines to replace a caliper or do other work. However, this is not a hard-and-fast rule, particularly if the brake line you are having work done to is independent of the other lines.

What happens if you don't bleed brakes in order? ›

What happens when air gets into the brake lines and if you don't bleed the brake system? You won't have responsive brakes. You will experience these issues: Spongy brakes.

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