Free CDL Air Brakes Questions and Answers

Question 1

To check the slack adjusters on S Cam brakes, which of the following procedures should you follow?

Accepted Answer

Use gloves and pull hard on each slack adjuster

Answer

To check the adjustment of S-cam brake slack adjusters, you should first ensure the parking brakes are off and the wheels are chocked. Then, using gloves for safety, pull hard on each slack adjuster. If the push rod moves more than about one inch (2.5 cm), the brakes are out of adjustment and need to be tightened.

Question 2

What is the normal air pressure in an air brake system that has been fully charged?

Accepted Answer

125 psi

Answer

A fully charged air brake system typically operates with an air pressure of approximately 125 psi. This is the 'cut-out' pressure where the compressor governor stops the compressor from pumping air into the reservoirs, maintaining optimal pressure for effective braking.

Question 3

How should one go about testing the air leakage rate correctly?

Accepted Answer

Turn off the engine and release the parking brake

Answer

To correctly test the air leakage rate, the engine must be turned off to prevent the compressor from replenishing lost air. The parking brake should also be released (with wheels chocked) to allow for a more accurate assessment of leaks throughout the entire service brake system when the foot pedal is applied.

Question 4

What is brake stab?

Accepted Answer

Applying your brakes until the wheels lock up, releasing, and then reapplying when the wheels start rolling

Answer

Brake stab (or stab braking) is a technique used in vehicles without ABS to stop quickly on slippery surfaces. It involves applying the brakes fully until the wheels lock, then releasing them until the wheels start rolling again, and immediately reapplying. This allows for maximum braking force while maintaining some steering control.

Question 5

Why do hydraulic brakes stop more quickly than air brakes?

Accepted Answer

It takes longer for the air to flow through the lines

Answer

Hydraulic brakes generally stop more quickly than air brakes because hydraulic fluid is incompressible and transmits force almost instantly through the lines. Air, being compressible, takes a fraction of a second longer to travel through the lines and build pressure in the brake chambers, resulting in a slight delay compared to hydraulic systems.

Air Brake Practice Test

Air Brake Practice Test

Free CDL Air Brakes Questions and Answers