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CDL Practice Test: Air Brakes

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CDL Practice Test: Air Brakes

Air Brakes Questions

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Good Luck!

At what psi must the low air pressure warning indicator activate?
  • Before dropping below 80 psi
  • Before dropping below 60 psi
  • Before dropping below 40 psi
  • Before dropping below 20 psi
This is a question from page 43 - click here to look up the answer

Quote From Page 64 Of The Illinois CDL Manual:

A warning signal you can see must come on before the air pressure in the tanks falls below 60 psi (or one-half the compressor governor cutout pressure on older vehicles).

TruckingTruth's Advice:

On large busses, it is common for the low-pressure warning devices to signal at 80 to 85 psi, but that is an additional safety feature. Regulations require the warning to activate before 60 psi. This is very important to memorize not only for testing, but for everyday job duties.

Next
Which of the following statements about Brake Fade is incorrect?
  • As overheated drums expand, the brake shoes and linings have to move further to contact the drums, and the force of this contact also is reduced
  • Brake fade results from excessive heat causing chemical changes in the brake lining, which reduce friction and cause the expansion of the brake drums
  • While brake fade was once a very common problem, it is very rare with modern brake cooling mechanisms
  • Continued overuse may increase brake fade until the vehicle cannot be slowed down or stopped at all
This is a question from page 47 - click here to look up the answer

Quote From Page 68 Of The Illinois CDL Manual:

Brakes are designed so brake shoes or pads rub against the brake drum or discs to slow the vehicle. Braking creates heat, but brakes are designed to take a lot of heat. However, brakes can fade or fail from excessive heat caused by using them too much and not relying on the engine braking effect.


Excessive use of the service brakes results in overheating and leads to brake fade. Brake fade results from excessive heat causing chemical changes in the brake lining, which reduce friction and cause the expansion of the brake drums. As the overheated drums expand, the brake shoes and linings have to move further to contact the drums, and the force of this contact also is reduced. Continued overuse may increase brake fade until the vehicle cannot be slowed down or stopped at all.

Brake fade also is affected by adjustment. To safely control a vehicle, every brake must do its share of the work. Brakes out of adjustment will stop doing their share before those that are in adjustment. The other brakes can then overheat and fade and there will not be sufficient braking available to control the vehicle(s). Brakes can get out of adjustment quickly, especially when they are hot. Therefore, brake adjustment must be checked frequently.

TruckingTruth's Advice:

While engine brakes are a fantastic tool in helping to prevent brake fade, the number one cause of brake failure in hilly conditions is brake fade.

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Next
Which statement best describes the Application Pressure Gauge?
  • Tells you how much pressure is in the air tanks
  • Application pressure gauges must be installed on all commercial vehicles
  • Shows how much air pressure you are applying to the brakes
  • Let's you know when air pressure in the tanks are too low
This is a question from page 43 - click here to look up the answer

Quote From Page 64 Of The Illinois CDL Manual:

Application Pressure Gauge - This gauge shows how much air pressure you are applying to the brakes. (This gauge is not on all vehicles.) Increasing application pressure to hold the same speed means the brakes are fading. You should slow down and use a lower gear. The need for increased pressure also can be caused by brakes out of adjustment, air leaks or mechanical problems.

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Next
What is the proper braking technique on long or steep downgrades?
  • Apply the brakes just hard enough to feel a definite slowdown, when your speed has been reduced to approximately 5mph below your "safe" speed, release the brakes and repeat as necessary
  • Begin the downgrade at a slow enough speed that brakes will not be necessary during the entire grade
  • Apply the brakes enough to keep a constant speed during the entire downgrade
  • Fluctuate between "hard braking" and "soft braking" but never fully release the brakes during a downgrade
This is a question from page 47 - click here to look up the answer

Quote From Page 68 Of The Illinois CDL Manual:

Proper Braking Technique - Remember, the use of brakes on a long and/or steep downgrade is only a supplement to the braking effect of the engine. Once the vehicle is in the proper low gear, the following is the proper braking technique:

  • 1. Apply the brakes just hard enough to feel a definite slowdown.
  • 2. When your speed has been reduced to approximately 5 mph below your "safe" speed, release the brakes. (This brake application should last about 3 seconds.)
  • 3. When your speed has increased to your "safe" speed, repeat steps 1 and 2.
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Next
While inspecting the air compressor, you should:
  • Take the front cover off so you can look inside the vacuum seal
  • Visually check oil level inside the air compressor using a dip-stick
  • Unbolt the air compressor from the engine as visually inspecting it while attached is nearly impossible
  • Check the condition and tightness of the belt (if belt driven)
This is a question from page 44 - click here to look up the answer

Quote From Page 66 Of The Illinois CDL Manual:

Check air compressor drive belt if compressor is belt driven. If the air compressor is belt driven, check the condition and tightness of the belt. The belt should be in good condition.

TruckingTruth's Advice:

Some air compressors are gear driven, in which case, no belt will be present. Otherwise, be sure to check the belt to be sure it isn't frayed, cracked, off its tracks, or otherwise broken.

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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 4 psi in 1 minute
  • No more than 3 psi in 1 minute
  • No more than 1 psi in 1 minute
  • No more than 2 psi in 1 minute
This is a question from page 45 - click here to look up the answer

Quote From Page 66 Of The Illinois 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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Next
The following statements accurately describe Wedge Brakes, except:
  • Wedge brakes may have either a single brake chamber or two brake chambers
  • All wedge-type brakes are self-adjusting
  • The brake chamber push rod pushes a wedge directly between the ends of two brake shoes
  • All of these answers are accurate
This is a question from page 42 - click here to look up the answer

Quote From Page 64 Of The Illinois CDL Manual:

Wedge brakes: In these types of brakes, the brake chamber push rod pushes a wedge directly between the ends of two brake shoes. This shoves them apart and against the inside of the brake drum. Wedge brakes may have a single brake chamber or two brake chambers, pushing wedges in at both ends of the brake shoes. Wedge-type brakes may be self-adjusting or may require manual adjustment.

TruckingTruth's Advice:

Wedge brakes are derived from old technology and are very rarely used in modern commercial vehicles. However, you still may be asked a question or two about wedge brakes on your written exam.

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Next
At about what psi should the air compressor stop pumping air into the air tanks?
  • 110 psi
  • 115 psi
  • 100 psi
  • 125 psi
This is a question from page 45 - click here to look up the answer

Quote From Page 66 Of The Illinois 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.

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