Automobile & Light Truck Certification Test A5 BrakesHydraulic System Diagnosis and RepairEasy
A technician is evaluating a brake system with a Proportioning Valve. The purpose of this valve is to:
- AIncrease hydraulic pressure to the front brakes during heavy braking.
- BPrevent rear wheel lock-up during light braking conditions.
- CDistribute equal hydraulic pressure to all four wheels at all times.
- DReduce hydraulic pressure to the rear brakes under heavy braking.
Show answer & explanationAnswer & explanation
Correct answer: D. Reduce hydraulic pressure to the rear brakes under heavy braking.
The proportioning valve's primary function is to reduce hydraulic pressure to the rear brakes under heavy braking. This prevents premature rear wheel lock-up, which can lead to instability and loss of control, especially as weight transfers to the front of the vehicle during deceleration.
Why the other options are wrong
- A. The proportioning valve reduces pressure to the rear, it does not increase pressure to the front.
- B. While it prevents rear lock-up, its main action is under heavy braking, not specifically light braking. Rear lock-up is more prone under heavy braking due to weight transfer.
- C. The proportioning valve actively *prevents* equal pressure distribution under heavy braking to maintain stability.
Proportioning Valve
A hydraulic valve in the brake system that limits the amount of fluid pressure reaching the rear brakes during hard braking.
- Prevents premature rear wheel lock-up.
- Reacts to sudden pressure increases.
- Works by restricting fluid flow to the rear circuit after a certain pressure threshold.
Memory trick: Proportioning: 'P' for Prevent rear lock-up.