CDL Air Brakes TestAir Brake System PartsMedium
A driver is performing a pre-trip inspection on a commercial vehicle. They notice that when the parking brake control knob is pushed in, the spring brakes do not release. What is the most likely cause of this issue?
- AThe anti-lock braking system (ABS) has detected an error.
- BThe front brake limiting valve is set to the 'slippery' position.
- CThere is a leak in the parking brake air line or control valve.
- DThe service brake chambers are faulty.
Show answer & explanationAnswer & explanation
Correct answer: C. There is a leak in the parking brake air line or control valve.
Spring brakes are held in the 'released' position by air pressure. If pushing the parking brake control knob (which sends air to release them) does not release them, it indicates a failure to deliver or maintain that air pressure, most commonly due to a leak in the parking brake air line or the control valve itself.
Why the other options are wrong
- A. An ABS error typically affects dynamic braking control, not the static release mechanism of spring brakes.
- B. The front brake limiting valve affects the air pressure to the front service brakes, not the release of the spring (parking) brakes.
- D. Faulty service brake chambers would affect service braking, not directly prevent spring brake release.
Spring Brakes
Powerful springs that apply the brakes when air pressure is lost or intentionally released (parking brake). They are held 'off' by air pressure.
- Applied by springs, released by air pressure.
- Act as parking and emergency brakes.
- Will engage automatically if air pressure drops too low.
Memory trick: No Air, No Go.