ELDT ELDT - Entry Level Driver Training Air Brakes Questions and Answers 2 — Questions and Answers
Question 1: What is the function of the dual air brake system in a commercial motor vehicle?
- To double braking force on the front axle
- To provide a backup system if one circuit fails (Correct answer)
- To allow independent trailer braking
- To cool brake drums faster
Correct answer: To provide a backup system if one circuit fails
A dual air brake system has two separate circuits so that if one fails, the other still provides enough pressure to stop the vehicle.
Question 2: How should a driver test the low air pressure warning device before driving?
- Drive until the alarm sounds
- Fan the brakes with the engine off until pressure drops to 60 psi (Correct answer)
- Apply full service brakes for 30 seconds
- Check it using a pressure gauge only
Correct answer: Fan the brakes with the engine off until pressure drops to 60 psi
With the engine off, repeatedly apply the brakes to reduce air pressure until the warning activates, which should occur at or above 60 psi.
Question 3: What happens to spring brakes when air pressure in the parking brake circuit falls below approximately 20–45 psi?
- They release automatically
- They apply automatically (Correct answer)
- They lock in neutral position
- Nothing — they require manual activation
Correct answer: They apply automatically
When air pressure drops below the spring brake activation threshold (approximately 20–45 psi), the springs push the brakes on automatically.
Question 4: What is a slack adjuster and why is it important in air brakes?
- A valve that controls airflow to the brake chamber
- A mechanical linkage that adjusts the distance between the brake lining and drum (Correct answer)
- A sensor that monitors brake pad thickness
- A pressure relief valve in the air tank
Correct answer: A mechanical linkage that adjusts the distance between the brake lining and drum
Slack adjusters connect the pushrod to the camshaft and must be properly adjusted to ensure the brake lining contacts the drum with adequate force.
Question 5: Which control is used to apply the trailer brakes independently of the tractor brakes?
- The foot pedal
- The hand valve (trailer brake) (Correct answer)
- The parking brake knob
- The emergency brake switch
Correct answer: The hand valve (trailer brake)
The hand valve (sometimes called the trolley valve) applies only the trailer brakes and is used for testing trailer brake function.
Question 6: What does an S-cam drum brake system use to spread the brake shoes against the drum?
- Air pressure directly
- A wedge mechanism
- An S-shaped camshaft (Correct answer)
- Hydraulic pistons
Correct answer: An S-shaped camshaft
In an S-cam brake, the rotating S-shaped cam forces the brake shoes outward against the drum when the brakes are applied.
ELDT ELDT - Entry Level Driver Training Air Brakes Questions and Answers 2
What is the function of the dual air brake system in a commercial motor vehicle?