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5.3 Inspecting Air Brake Systems (continued)

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:

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 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.

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 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...

The air compressor should start pumping air into the air tanks when pressure drops below what psi?
  • 115 psi
  • 110 psi
  • 125 psi
  • 100 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.

Next
With a fully charged air system and the service brake released, what is the maximum allowable air loss rate?
  • Less than 3 psi in 1 minute for single vehicles and less than 2 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 2 psi in 1 minute for single vehicles and less than 3 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
At what psi should the low air warning light and buzzer activate?
  • 100 psi
  • None of these are correct
  • 120 psi
  • 80 psi

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
At what psi will the air compressor governor tell the air compressor to "cut-in"?
  • 110 psi
  • 120 psi
  • 90 psi
  • 100 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 proper engine operating RPM, how quickly should air pressure build from 85 to 100 psi with a dual air system?
  • Within 30 seconds
  • Within 25 seconds
  • Within 45 seconds
  • Within 1 minute

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
At about what psi should the air compressor stop pumping air into the air tanks?
  • 110 psi
  • 115 psi
  • 125 psi
  • 100 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
If the air governor stops working properly, which of the following could occur?
  • You could encounter steering difficulties
  • All of these answers are correct
  • The air system may not keep enough air pressure for safe driving
  • You may get increased condensation in your 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.

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
At what psi will the air compressor governor tell the air compressor to "cut out"?
  • About 110 psi
  • About 90 psi
  • About 115 psi
  • About 125 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 rate for a combination vehicle when 90 psi or more is applied with the brake pedal?
  • No more than 2 psi in 1 minute
  • No more than 1 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
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
What is the proper way to test the service brakes?
  • Service brakes should only be tested by a certified mechanic
  • Chock the wheels, turn off the engine, then release all brakes to perform a manual inspection of the service brakes
  • Move the vehicle forward slowly (about 5 mph) and apply the brakes firmly using the brake pedal, making note of any unusual stopping action
  • 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

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.

Prev
Next
How can you check that the spring brakes come on automatically?
  • 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 disconnect a brake line and allow the air to deplete which will show if the spring brakes activate 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
  • 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

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.

Prev
Next
What is "Cut-In Air Pressure"?
  • 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 emergency brakes to engage
  • 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
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 4 psi in 1 minute for single vehicles and less than 5 psi in 1 minute for combination vehicles
  • None of these answers are correct
  • 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 the maximum allowable air loss for a single vehicle with the service brake applied?
  • No more than 3 psi in 1 minute
  • No more than 4 psi in 1 minute
  • No more than 5 psi in 1 minute
  • No more than 6 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
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 3 psi in 1 minute for single vehicles and less than 4 psi in 1 minute for combination vehicles
  • None of these answers are correct
  • 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
  • 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

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
Which of the following steps to checking the low pressure warning signal is incorrect?
  • Step 1: Shut the engine off when you have enough air pressure that the low-pressure warning signal is not on
  • All of these steps are correct
  • 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
  • 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 should you properly test the 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
  • Place chocks against the tires of the truck, release the parking brake, then visually inspect each 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
  • 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.

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Next
What is the maximum allowable air loss rate for a combination vehicle with the service brakes released?
  • Less than 1 psi in 1 minute
  • Less than 2 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!!

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Becoming A Truck Driver

Becoming A Truck Driver is a dream we've all pondered at some point in our lives. We've all wondered if the adventure and challenges of life on the open road would suit us better than the ordinary day to day lives we've always known. At TruckingTruth we'll help you decide if trucking is right for you and help you get your career off to a great start.

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