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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!

Why does brake fade occur?
  • Brake fade occurs when brakes become too cold during freezing weather conditions
  • Brake fade occurs when excessive moisture builds onto the brake linings
  • Brake fade occurs when using the engine brake for too long on a steep downgrades
  • Brakes can fade or fail from excessive heat caused by using them too much and not relying on the engine braking effect
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:

Brake fade is the #1 cause of all brake failures during mountain driving. Most trucks are equipped with engine braking devices which should be used as often as possible.

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What does the Air Compressor Governor do?
  • Controls when the air compressor will pump air into the air storage tanks
  • Detects any leaks in the airlines
  • Ensures that air never stops flowing into the air storage tanks
  • Activates the emergency brakes when air pressure gets too high
This is a question from page 41 - click here to look up the answer

Quote From Page 62 Of The Illinois CDL Manual:

The governor controls when the air compressor will pump air into the air storage tanks. 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.

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What is a Modulating Control Valve?
  • A valve used to apply the service brakes
  • A brake valve which allows the truck to roll back, but not forwards
  • A control handle on the dash board may be used to apply the spring brakes gradually
  • A valve which allows you to use the spring brakes for everyday driving
This is a question from page 43 - click here to look up the answer

Quote From Page 65 Of The Illinois CDL Manual:

Modulating control valves: In some vehicles a control handle on the dash board may be used to apply the spring brakes gradually. This is called a "modulating valve." It is spring-loaded so you have a feel for the braking action. The more you move the control lever, the harder the spring brakes come on. They work this way so you can control the spring brakes if the service brakes fail. When parking a vehicle with a modulating control valve, move the lever as far as it will go and hold it in place with the locking device.

TruckingTruth's Advice:

Very few vehicles are equipped with modulating control valves. But since there are some air brake vehicles with this valve, you may be asked a question about it on the written exam.

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When should air tanks be drained?
  • Every month
  • At the end of each day
  • None of these answers are correct
  • At least once per week
This is a question from page 47 - click here to look up the answer

Quote From Page 69 Of The Illinois CDL Manual:

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.

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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
  • 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
  • Place chocks against the tires of the truck, release the parking brake, then visually inspect each parking brake
This is a question from page 45 - click here to look up the answer

Quote From Page 67 Of The Illinois 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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Which of the following statements about the Air Compressor is false?
  • Air compressors may have their own oil supply or be lubricated by engine oil
  • The air compressor pumps air into the air storage tanks
  • The air compressor is connected to the engine through gears or a V-belt
  • Air compressors are always air cooled
This is a question from page 41 - click here to look up the answer

Quote From Page 62 Of The Illinois CDL Manual:

The air compressor pumps air into the air storage tanks (reservoirs). The air compressor is connected to the engine through gears or a V-belt. The compressor may be air cooled or may be cooled by the engine cooling system. It may have its own oil supply or be lubricated by engine oil. If the compressor has its own oil supply, check the oil level before driving.

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Spring brakes should activate when air pressure drops to:
  • Between 50 and 75 psi
  • Between 100 and 120 psi
  • Between 20 and 45 psi
  • Between 10 and 20 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 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 1 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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