CDL Air Brakes TestAir Brake System PartsMedium
A commercial vehicle driver notices that during moderate braking, the front wheels tend to lock up prematurely, especially on slippery surfaces. The vehicle is equipped with a front brake limiting valve. What is the likely cause of this issue?
- AThe spring brakes are partially engaged, reducing braking effectiveness.
- BThe front brake limiting valve is stuck in the 'slippery' position.
- CThe front brake limiting valve is stuck in the 'normal' position.
- DThe air compressor governor is set too high, causing excessive front brake pressure.
Show answer & explanationAnswer & explanation
Correct answer: C. The front brake limiting valve is stuck in the 'normal' position.
The front brake limiting valve, when used, has a 'normal' setting (full pressure to front brakes) and a 'slippery' setting (reduced pressure to front brakes). If the front wheels lock up prematurely on slippery surfaces, it suggests the valve is providing too much pressure, meaning it is stuck in or set to the 'normal' position when it should be reducing pressure.
Why the other options are wrong
- A. Partially engaged spring brakes would reduce overall braking effectiveness, but wouldn't specifically cause front wheel lock-up during moderate braking.
- B. If stuck in 'slippery' mode, the front brakes would have reduced power, making them less likely to lock up prematurely.
- D. The governor affects overall system pressure, not the differential pressure to specific axles or the function of a limiting valve.
Front Brake Limiting Valve
A control valve, sometimes found on older commercial vehicles, that allows the driver to reduce the air pressure to the front brakes by about half when driving on slippery surfaces, to prevent front wheel lock-up.
- Reduces front brake pressure for slippery conditions.
- Typically has 'normal' and 'slippery' settings.
- Designed to prevent steering loss due to front wheel lock-up.
Memory trick: Slippery Surface, Front Locks? Limiter's Not Limiting!