CDL Air Brakes TestAir Brake System PartsMedium
A driver notices that when they release the parking brake control knob, the spring brakes on the vehicle do not fully release, causing the vehicle to drag. What is the most likely cause of this issue?
- AA malfunctioning low air pressure warning system.
- BInsufficient air pressure reaching the spring brake chambers.
- CThe service brake chambers are leaking.
- DA faulty air compressor governor.
Show answer & explanationAnswer & explanation
Correct answer: B. Insufficient air pressure reaching the spring brake chambers.
Spring brakes are held in the 'off' position by air pressure. If the parking brake control knob is released but the spring brakes don't fully release, it indicates that insufficient air pressure is reaching the spring brake chambers to overcome the spring force. This could be due to a restriction, leak, or faulty parking brake control valve itself.
Why the other options are wrong
- A. A malfunctioning low air warning system would not directly cause spring brakes to drag; it would fail to warn of low pressure.
- C. Leaking service brake chambers would affect service brake application, not the release mechanism of the spring (parking) brakes.
- D. A faulty air compressor governor affects the overall system pressure (cut-in/cut-out), not specifically the release of spring brakes unless the overall system pressure is too low.
Parking Brake Control Valve & Spring Brakes
The parking brake control valve controls the flow of air pressure to the spring brake chambers. When released, it allows air pressure to push against the spring, releasing the brakes.
- Spring brakes are 'spring-applied, air-released'.
- Air pressure overcomes spring force to release brakes.
- Insufficient air pressure or a faulty valve prevents full release.
Memory trick: Spring brakes stuck? Air pressure's low, or the valve won't flow!