CLP Air Brake Systems 3 — Questions and Answers
Question 1: What is the correct technique for braking on a long, steep downgrade with air brakes?
- Apply brakes hard at the top and ride them all the way down
- Use a light, steady brake application throughout the descent
- Apply brakes firmly to reduce speed by about 5 mph, release, then reapply as needed (Correct answer)
- Put the transmission in neutral to reduce engine resistance and brake continuously
Correct answer: Apply brakes firmly to reduce speed by about 5 mph, release, then reapply as needed
The proper technique is to apply brakes firmly until speed drops about 5 mph below your target speed, then release them and allow brakes to cool before reapplying.
Question 2: What does a properly functioning air dryer do in an air brake system?
- Cools the compressed air before it enters the brake chambers
- Removes moisture and contaminants from compressed air before it reaches the tanks (Correct answer)
- Increases air pressure to compensate for system leaks
- Automatically drains the air tanks every 8 hours
Correct answer: Removes moisture and contaminants from compressed air before it reaches the tanks
An air dryer removes water vapor and contaminants from compressed air to prevent moisture buildup that could freeze lines or damage system components.
Question 3: How much air pressure loss per minute is acceptable for a combination vehicle during the static leakage test with brakes released?
- 3 psi or less
- 5 psi or less (Correct answer)
- 1 psi or less
- 8 psi or less
Correct answer: 5 psi or less
For a combination vehicle with brakes released, the air pressure should not drop more than 4 psi in one minute (3 psi for single vehicles); 5 psi or less is acceptable.
Question 4: What is the role of the S-cam in a drum air brake system?
- It regulates the amount of air sent to each brake chamber
- It rotates to force the brake shoes outward against the brake drum when brakes are applied (Correct answer)
- It connects the air chamber push rod to the slack adjuster
- It prevents the brakes from locking up during hard stops
Correct answer: It rotates to force the brake shoes outward against the brake drum when brakes are applied
When air pressure moves the push rod, it turns the S-cam, which pushes the brake shoes outward against the drum to create friction and slow the vehicle.
Question 5: Why should you never push down the brake pedal to release spring brakes (parking brakes) when there is no air pressure?
- It will damage the foot valve and require expensive repairs
- There is no air pressure to compress the springs, so the service brake pedal cannot release them (Correct answer)
- Doing so will cause the trailer brakes to apply simultaneously
- It bypasses the ABS system and creates a skid risk
Correct answer: There is no air pressure to compress the springs, so the service brake pedal cannot release them
Spring brakes are held released by air pressure; without air, the springs are fully extended and the foot valve cannot mechanically compress them.
Question 6: What is a slack adjuster, and when does it need adjustment?
- A valve that adjusts air pressure to match vehicle load; adjusted at every oil change
- A mechanical arm that adjusts the distance between brake shoes and drum; needs adjustment when push rod travel exceeds limits (Correct answer)
- An electronic sensor that monitors brake pad wear; replaced when warning light activates
- A spring-loaded device that adjusts pedal height for different drivers
Correct answer: A mechanical arm that adjusts the distance between brake shoes and drum; needs adjustment when push rod travel exceeds limits
Slack adjusters connect the push rod to the S-cam and compensate for brake lining wear; they must be adjusted when push rod stroke exceeds the legal limit.
Question 7: What happens to the emergency brakes on a trailer if the trailer breaks away from the tractor?
- Nothing — trailer brakes are controlled only from the cab
- The trailer's spring brakes automatically apply due to the loss of air supply (Correct answer)
- The trailer coasts freely until it comes to rest
- The trailer ABS system activates and brings it to a controlled stop
Correct answer: The trailer's spring brakes automatically apply due to the loss of air supply
When a trailer breaks away, the air supply line loses pressure, causing the trailer's spring brakes to automatically apply and stop the runaway trailer.
CLP Air Brake Systems 3
What is the correct technique for braking on a long, steep downgrade with air brakes?