Brake Systems — Function of the Proportioning Control Valve (PCV) In a hydraulic service-brake system, what does the proportioning control valve do when line pressure exceeds a calibrated threshold?

Introduction / Context:
Under heavy braking, weight shifts forward, greatly increasing the front axle’s normal load while unloading the rear axle. Brake system designers use proportioning to prevent premature rear-wheel lock, which can cause instability. This question focuses on the specific action of the proportioning control valve (PCV).

Given Data / Assumptions:

• Typical passenger car with front-disc/rear-drum or rear-disc brakes.
• Hydraulic split with a mechanical or pressure-sensitive PCV.
• High pedal effort where line pressure rises above a set “knee” point.

Concept / Approach:
A PCV is a pressure-sensitive device that reduces the rate of pressure rise to the rear circuit after a threshold. Below the knee, front and rear pressures rise nearly 1:1. Above it, the rear circuit sees a reduced slope so that rear brakes develop less torque than a simple 1:1 system would, matching available tire–road friction at the rear.

Step-by-Step Solution:
Identify vehicle dynamics: forward load transfer makes rear tires easier to lock.Control objective: limit rear pressure at high deceleration.Device action: above threshold, the PCV “proportions” so rear pressure < front pressure.Therefore select the option describing reduced rear pressure at higher line pressures.

Verification / Alternative check:
Brake balance charts show a kinked rear line beyond the knee pressure; lab tests confirm delayed rear lock compared to an un-proportioned system.

Why Other Options Are Wrong:
Equal pressure at all times: contradicts proportioning function.Generic “reduce pressure near lockup”: that is the job of ABS modulators, not a fixed PCV.Less pressure to front: opposite of safe biasing under weight transfer.Bypass to reservoir: not a PCV function.

Common Pitfalls:
Confusing the PCV with ABS; PCV is passive and pressure-sensitive, ABS is active and wheel-speed-based.

Final Answer:
cause less fluid pressure to act on the rear brakes than the front brakes when the fluid pressure exceeds a predetermined level