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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!

In a vehicle equipped with an air brake system, what should happen when air pressure drops below 60 psi
  • The emergency brakes should activate
  • None of these answers are correct
  • The service brakes will begin to slowly apply brake pressure
  • Nothing, 60 psi is considered "normal operating range"
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:

Before air pressure drops below 60 psi, the low air warning signal will activate. When the warning activates, you need to quickly find a safe place to pull your vehicle to the side of the road. If you wait too long, the emergency brakes will activate once your air pressure reaches 45 psi and you may not be able to clear the roadway.

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 1 psi in 1 minute
  • No more than 4 psi in 1 minute
  • No more than 3 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
Spring brakes should activate when air pressure drops to:
  • Between 10 and 20 psi
  • Between 100 and 120 psi
  • Between 20 and 45 psi
  • Between 50 and 75 psi
This is a question from page 47 - click here to look up the answer

Quote From Page 68 Of The Illinois CDL Manual:

The spring brakes will come on when the air pressure drops into the 20 to 45 psi range

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What is a "safe speed"?
  • 10mph under the posted speed limit is considered a safe speed
  • Speeds of 55mph or below are considered safe speeds for maximum fuel mileage
  • The yellow signs before curves and ramps are designated safe speeds
  • A predetermined speed for descending down a long or steep grade, which helps determine when brakes should be applied or released
This is a question from page 47 - click here to look up the answer

Quote From Page 68 Of The Illinois CDL Manual:

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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The following are all true about parking brakes, except:
  • When there are freezing temperatures and the brakes are very wet, the brake linings may freeze to the brake drums
  • All of these answers are true
  • When the brakes are very hot (i.e. just after coming down a steep grade) wheel chocks should be used instead of the parking brake
  • Older vehicles may have a round blue knob for the parking brake in place of the yellow diamond shaped knob currently used in modern equipment
This is a question from page 47 - click here to look up the answer

Quote From Page 68 Of The Illinois CDL Manual:

Any time you park, use the parking brakes, except as noted. Pull the parking brake control knob out to apply the parking brakes. Push it in to release them. The control will be a yellow, diamond-shaped knob labeled "parking brakes" on newer vehicles. On older vehicles, it may be a round blue knob or some other shape, including a lever that swings from side to side or up and down.

Do not use the parking brakes if the brakes are very hot (i.e., from just coming down a steep grade), or if the brakes are very wet in freezing temperatures. If the brakes are used when very hot, they can be damaged by the heat. If they are used in freezing temperatures when the brakes are very wet, they can freeze so the vehicle cannot move. Use wheel chocks to hold the vehicle. Let hot brakes cool before using the parking brakes. If the brakes are wet, use the brakes lightly while driving in a low gear to heat and dry them.

If your vehicle does not have automatic air tank drains, drain your air tanks at the end of each working day to remove moisture and oil. Otherwise, the brakes could fail.


Never leave your vehicle unattended without applying the parking brakes or chocking the wheels. Your vehicle might roll away and cause injury and damage

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Next
How can Brake Fade be avoided?
  • Properly calibrating slack adjusters
  • All of these are ways to help avoid Brake Fade
  • Avoiding excessive use of the service brakes
  • Making sure all brake linings are in good condition
This is a question from page 47 - click here to look up the answer

Quote From Page 68 Of The Illinois CDL Manual:

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.

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Next
At what pressure will the air compressor governor tell the air compressor to "cut out"?
  • Around 100 psi
  • Around 125 psi
  • Around 75 psi
  • Around 150 psi
This is a question from page 41 - click here to look up the answer

Quote From Page 62 Of The Illinois CDL Manual:

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.

TruckingTruth's Advice:

Be sure to understand that the air compressor governor has one main purpose and that is to tell the air compressor when to pump air and when to stop pumping air. The air compressor governor will tell the air compressor to start supplying air (cut in) and when to stop supplying air (cut out).

  • Cut in air pressure is usually when air pressure falls to about 100 psi.
  • Cut out air pressure is usually when air pressure raises to about 125 psi
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In a Dual Air Brake System:
  • The low air warning should activate before 120 psi since it's a dual air brake system
  • The low air warning should not activate until 30 psi since there are two air brake systems
  • The low air warning should only activate when both systems drop below 60 psi
  • The low air warning should activate before pressure drops below 60 psi in either system
This is a question from page 44 - click here to look up the answer

Quote From Page 65 Of The Illinois CDL Manual:

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. This means it will take you longer to stop. Bring the vehicle to a safe stop and have the air brake system fixed.

TruckingTruth's Advice:

Regardless of whether your vehicle has a single air brake system or a duel air brake system, if any air pressure gauge reads below 60 psi, you should have a warning about low air pressure even if the second system is fully charged.

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