CDL General Knowledge TestAir Brakes (if applicable)Medium
A technician is troubleshooting a vehicle with air brakes. They find that the parking brake will not release, even though there is sufficient air pressure in the system. Which component is the most likely cause of this issue?
- AA clogged air dryer.
- BExcessive slack in the automatic slack adjusters.
- CA malfunctioning air compressor.
- DA stuck or faulty parking brake control valve.
Show answer & explanationAnswer & explanation
Correct answer: D. A stuck or faulty parking brake control valve.
The parking brake control valve is responsible for allowing air to flow to or from the spring brake chambers, controlling the application and release of the parking brake. If it's stuck or faulty, it can prevent air from reaching the chambers to release the brakes, even with adequate system pressure.
Why the other options are wrong
- A. A clogged air dryer affects air quality and build-up, not specifically the release of the parking brake when pressure is present.
- B. Slack adjuster issues affect service brake effectiveness, not parking brake release.
- C. A faulty air compressor would prevent air from building, not prevent release when pressure is sufficient.
Parking Brake Control Valve
A manual valve (often a push-pull knob) that controls the flow of air to the spring brake chambers, engaging or releasing the parking brakes.
- Pushed in to release parking brakes (air supplied).
- Pulled out to apply parking brakes (air exhausted).
- A common point of failure for parking brake issues.
Memory trick: Valve Vexes Release: If the valve won't move, neither will the brake.