CDL Air Brakes TestAir Brake System PartsMedium
A driver observes that when they release the parking brake control knob, the spring brakes do not release. The supply pressure gauges indicate full system pressure (125 psi). What is the most likely cause?
- AThe parking brake control knob or its valve is faulty.
- BThe low air pressure warning system is malfunctioning.
- CThe service brakes are seized due to overheating.
- DThe air compressor governor is stuck in the 'cut-out' position.
Show answer & explanationAnswer & explanation
Correct answer: A. The parking brake control knob or its valve is faulty.
The parking brake control knob operates a valve that sends air pressure to release the spring brakes. If the knob is released but the spring brakes remain applied despite having full system air pressure, the valve itself (part of the control knob assembly) is likely faulty and not allowing air to reach the spring brake chambers.
Why the other options are wrong
- B. The low air pressure warning system is irrelevant when the system has full pressure.
- C. Seized service brakes would affect driving, but not prevent spring brakes from releasing if air is supplied.
- D. A governor stuck in cut-out would prevent pressure from building, but here, there is full pressure.
Parking Brake Control Valve
A valve operated by the parking brake control knob that either applies air pressure to hold spring brakes released or exhausts air to allow spring brakes to apply.
- Controls air flow to spring brakes.
- Pushing in releases spring brakes (applies air).
- Pulling out applies spring brakes (exhausts air).
Memory trick: Knob's Command: Air Flow, Release the Hold!