CDL Air Brakes TestAir Brake System PartsHard
A technician is troubleshooting a commercial vehicle's air brake system. They observe that when the service brakes are applied, one of the brake chambers has a significantly longer pushrod stroke than the others, exceeding the maximum allowable limit. What is the most likely consequence of this issue?
- AThe low air pressure warning will activate prematurely.
- BThe air compressor will cycle more frequently.
- CThe foundation brake at that wheel will be out of adjustment and less effective.
- DThe brake pedal will feel firmer than usual.
Show answer & explanationAnswer & explanation
Correct answer: C. The foundation brake at that wheel will be out of adjustment and less effective.
An excessively long pushrod stroke indicates that the slack adjuster for that brake is out of adjustment, or the brake linings are severely worn. This means the brake shoes are not contacting the drum effectively, leading to reduced braking force at that wheel, potential overheating, and uneven braking.
Why the other options are wrong
- A. Premature low air warning is usually due to a systemic leak or compressor issue, not a single brake's adjustment unless the leak is substantial through that chamber.
- B. While a leak in the chamber could cause this, excess stroke itself doesn't directly cause frequent compressor cycling unless it's a significant air leak.
- D. A long pushrod stroke would typically lead to a softer or 'spongy' pedal due to more air being used and more travel required, not a firmer one.
Foundation Brakes (Pushrod Stroke)
The distance the brake chamber's pushrod moves when the service brakes are fully applied. This stroke directly indicates the adjustment of the foundation brake; excessive stroke means the brake is out of adjustment.
- Measured during a brake test.
- Maximum allowable stroke varies by brake chamber size.
- Excessive stroke leads to reduced braking power and potential brake fade.
Memory trick: Long Stroke, Weak Brake.