ELDT - Entry Level Driver Training Air Brakes Questions and Answers Flashcards

What is the function of the dual air brake system in a commercial motor vehicle?

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.

How should a driver test the low air pressure warning device before driving?

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.

What happens to spring brakes when air pressure in the parking brake circuit falls below approximately 20–45 psi?

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.

What is a slack adjuster and why is it important in air brakes?

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.

Which control is used to apply the trailer brakes independently of the tractor brakes?

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.

What does an S-cam drum brake system use to spread the brake shoes against the drum?

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.