CDL Air Brakes TestAir Brake System PartsHard
A technician is diagnosing an issue where a commercial vehicle's spring brakes are not releasing despite adequate air pressure in the system and the parking brake control knob being pushed in. The air compressor is building and maintaining proper pressure. What component is the most likely culprit?
- AA defective parking brake control valve.
- BA faulty air compressor governor.
- CA clogged air dryer.
- DA seized S-cam in the foundation brakes.
Show answer & explanationAnswer & explanation
Correct answer: A. A defective parking brake control valve.
If the spring brakes aren't releasing, but there's enough air pressure in the system and the parking brake knob is 'in' (signaling release), it means the air isn't reaching the spring brake chambers or isn't being properly directed to compress the springs. The parking brake control valve is responsible for allowing air into (to release) and out of (to apply) the spring brake chambers. A defect here would prevent release.
Why the other options are wrong
- B. A faulty governor would affect the compressor's ability to build/maintain pressure, which is stated as 'adequate'.
- C. A clogged air dryer primarily affects moisture removal and air flow, not the specific control of spring brake release when system pressure is adequate.
- D. A seized S-cam would prevent the service brakes from applying, but wouldn't prevent the spring brakes from releasing; the spring brake acts on the pushrod, which then acts on the S-cam.
Parking Brake Control Valve
A valve, typically located on the dashboard, that controls the flow of air to the spring brake chambers, allowing the driver to apply or release the parking/emergency brakes.
- When pulled out, it exhausts air from spring brake chambers, applying brakes.
- When pushed in, it allows air into spring brake chambers, releasing brakes.
- A critical component for controlling spring brake function.
Memory trick: Knob In, Air High, Still Stuck, Valve's Why.