Free CDL Air Brakes Endorsement II Practice Test Questions

  1. What is a dual air brake system?
  1. A two-brake system where one operates the front brakes and one operates the back brakes
  2. A two-brake system (one for each side of the vehicle)
  3. A two-brake system (one for daytime and one for nighttime driving)
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The correct answer is A!

A dual air brake system consists of two separate, independent air brake systems that use a single set of brake controls. Typically, one system (the “primary”) operates the brakes on the rear axles, and the other system (the “secondary”) operates the brakes on the front axle. If one system fails due to an air leak, the other system remains operational, providing a partial but crucial braking ability.

 

  1. What are slack adjusters?
  1. The space between the back wheels
  2. The lever that connects the brake chamber pushrod to the S-cam
  3. An adjusting nut under the cab to adjust the springs
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The correct answer is B!

A slack adjuster is a lever arm in a drum brake system that connects the brake chamber’s pushrod to the S-cam. When you press the brake, the pushrod moves the slack adjuster, which turns the S-cam and expands the brake shoes.

 

  1. How can you check the slack adjuster?
  1. Park on level ground, chock the wheels, release the parking brakes, and pull firmly on each slack adjuster.
  2. Park on level ground, release the parking brake, and press the brake pedal.
  3. Park on an incline and see if the vehicle rolls.
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The correct answer is A!

To check a manual slack adjuster for proper free play, you must park on level ground, chock the wheels to prevent rolling, and release the parking brakes. This allows the pushrod to move freely.

You then go under the vehicle and physically pull on the slack adjuster arm. If it moves more than approximately one inch, it is out of adjustment and needs to be tightened.

 

  1. How can you test the low air pressure warning signal?
  1. While the engine is running, step on and off the brake pedal
  2. Shut off the engine and see if the air pressure leaks down
  3. Shut off the engine with the system charged, turn the key ON, then pump the brakes until the warning activates
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  1. How can you check that the spring brakes come on automatically?
  1. With the system charged and engine OFF, and with the key ON, fan the brakes after the low-air warning comes on until the parking brake valve pops out
  2. Get under the vehicle and pull on the spring brakes
  3. Start the engine and hold the brake pedal to the floor
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  1. What are the maximum leakage rates?
  1. <4 psi in one minute for single vehicles, and <8 psi in one minute for combination vehicles
  2. <2 psi in one minute for single vehicles, and <3 psi in one minute for combination vehicles
  3. <6 psi for all vehicles
Show Answer
The correct answer is B!

With the air system fully charged, engine off, and parking brake released, the system should not lose more than 2 psi in one minute for a single vehicle or 3 psi in one minute for a combination vehicle.

 

  1. Should you ever leave your vehicle unattended without applying parking brakes or chocking the wheels?
  1. Yes
  2. No
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The correct answer is B!

This is a fundamental safety rule. Air pressure can slowly leak from the system, which could lead to a vehicle rolling away if it’s on an incline and not secured. Always set the parking brakes and chock the wheels if you are on any grade.

 

  1. If you are away from your vehicle only a short time, you don’t need to use the parking brake.
  1. True
  2. False
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The correct answer is B!

You must apply the parking brake every time you leave the vehicle, no matter how short the duration. A vehicle can begin to roll in an instant.

 

  1. How often should you drain air tanks?
  1. Daily
  2. Weekly
  3. Monthly
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The correct answer is A!

Air tanks should be drained daily, typically at the end of each day’s shift. The air compressor pumps air, moisture, and a tiny amount of oil into the tanks. This sludge must be drained to prevent it from freezing in cold weather or gumming up valves and seals.

 

  1. The use of brakes on a long steep downgrade is only a supplement to the braking effect of the engine.
  1. True
  2. False
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The correct answer is A!

The primary braking force on a long downgrade should come from engine braking. The service brakes should only be used intermittently to slow the vehicle when its speed increases.

 

  1. What factors can cause brakes to fade or fail?
  1. Not pressing the brake pedal hard enough
  2. Overheating, low air pressure, and not relying on the engine braking effect
  3. Not taking your foot off the accelerator pedal
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The correct answer is B!

Brake fade is the loss of braking power when the brakes get too hot. This is most often caused by overheating from excessive use on a downgrade.

Brakes can also fail due to low air pressure, which means they cannot be applied with sufficient force, or mechanical failures caused by water or oil in the lines.

 

  1. Why should you be in the proper gear before starting down a hill?
  1. So you only have to apply the brake just hard enough to feel a definite slowdown
  2. So you don’t have to shift gears going downhill
  3. So you can go through the gears on the way down
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The correct answer is B!

You must select the proper low gear before starting the downgrade because it is extremely difficult, and often impossible, to shift into a lower gear once the vehicle has built up speed.

If you try to shift and miss the gear, you will lose all engine braking effect, and the vehicle will rapidly accelerate. This forces you to rely entirely on your service brakes, which will quickly overheat and fail.

 

  1. If oil and water collect in the air tanks, what can happen to the brakes?
  1. The brakes could heat up.
  2. The brakes could fail.
  3. The brakes could lock up.
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The correct answer is B!

In cold weather, this moisture can freeze inside air lines or valves, blocking the flow of air and causing the brakes to fail. The sludge can also degrade rubber seals or gum up valves, leading to air leaks and brake failure at any temperature.

 

  1. What is the purpose of an alcohol evaporator?
  1. To reduce the frequency of draining the air tanks
  2. To have a drink available if you get thirsty
  3. To reduce the risk of ice in the brake valves and other parts in cold weather
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The correct answer is C!

An alcohol evaporator automatically injects a small amount of alcohol vapor into the air system. This alcohol mixes with the moisture in the air and acts as an antifreeze, preventing the water from freezing inside the air lines and valves, which would otherwise cause the brakes to fail.

 

  1. Which term is commonly used to describe applying firm, steady pressure to the brake pedal?
  1. Steady braking
  2. Squeeze braking
  3. Hard braking
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The correct answer is B!

Controlled braking (squeeze braking) involves applying the brake pedal with firm, steady, and progressive pressure (like squeezing an orange).