CDL Air Brakes TestAir Brake System PartsMedium
During a pre-trip inspection, a driver notices that when the service brakes are applied, the brake chambers on the drive axle extend, but the brake shoes do not move away from the drums. What is the most likely cause of this issue?
- AA misadjusted brake pedal free play.
- BLow air pressure in the secondary tank.
- CA clogged air dryer.
- DA faulty slack adjuster.
Show answer & explanationAnswer & explanation
Correct answer: D. A faulty slack adjuster.
If the brake chamber pushrod extends but the brake shoes don't move, it indicates a problem in the linkage between the pushrod and the brake shoes. A faulty or improperly adjusted slack adjuster is the most common cause, as it's responsible for transmitting the force from the pushrod to the cam and thence to the brake shoes.
Why the other options are wrong
- A. Brake pedal free play affects the timing of brake application, not the mechanical engagement of the brake shoes once the chamber activates.
- B. Low air pressure would prevent the pushrod from extending fully, which is not the issue described.
- C. A clogged air dryer would affect air supply, not the mechanical linkage of the foundation brakes.
Slack Adjuster Function
A component of the foundation brake system that adjusts the distance between the brake shoes and the drum, ensuring proper brake application and compensating for lining wear.
- Connects the brake chamber pushrod to the S-cam.
- Maintains correct brake shoe-to-drum clearance.
- Automatically or manually adjusts for brake lining wear.
Memory trick: Slack Adjuster Snags Shoes.