Automobile & Light Truck Certification Test A5 BrakesHydraulic System Diagnosis and RepairHard
A technician is evaluating a brake system that uses a combination valve. The vehicle exhibits normal braking under light pedal pressure, but when heavy braking is applied, the rear wheels tend to lock up prematurely. The front brakes seem to be doing most of the work. Which function of the combination valve is MOST likely failing?
- AThe pressure differential switch function.
- BThe residual pressure valve function.
- CThe metering valve function.
- DThe proportioning valve function.
Show answer & explanationAnswer & explanation
Correct answer: D. The proportioning valve function.
Premature rear wheel lock-up during heavy braking is a classic symptom of a faulty proportioning valve function. The proportioning valve is designed to reduce hydraulic pressure to the rear brakes under heavy braking to prevent lock-up, as weight transfers to the front. If this function is failing, the rear brakes receive too much pressure.
Why the other options are wrong
- A. The pressure differential switch illuminates the brake warning light if there's a pressure loss in one circuit. It doesn't affect braking balance or lock-up.
- B. The residual pressure valve maintains a small amount of pressure in the drum brake system to keep the shoes close to the drums. A fault here would affect pedal travel or shoe contact, not cause premature rear lock-up during heavy braking.
- C. The metering valve delays front brake engagement to allow rear shoes to contact drums first, preventing premature front lock-up. A fault here might affect light braking or cause front lock-up, not rear.
Proportioning Valve Function
The proportioning valve, often part of a combination valve, reduces hydraulic pressure to the rear brakes during heavy braking. This prevents premature rear wheel lock-up caused by the forward weight transfer, ensuring stable and controlled deceleration.
- Reduces rear brake pressure.
- Activates during heavy braking.
- Prevents rear wheel lock-up.
- Compensates for weight transfer.
- Ensures braking stability.
Memory trick: Rear wheels lock, the proportioning valve's clock is stuck!