What is a brake proportioning valve?

Definition & Description

A Brake Proportioning Valve is commonly found on vehicles with front disc and rear drum brakes. It is a safety valve that restricts flow to the rear brakes during a “panic stop.” This prevents the rear wheels from locking up before the front wheels.

An inline proportioning valve is plumbed into the system immediately after the Master Cylinder. In many vehicles, the proportioning valve is part of the Combination Valve.

How does it work?

The proportioning valve consists of a valve attached to a piston. During normal operation, the piston is held open by a spring. This allows fluid to flow freely to the rear brakes.

During sudden, hard braking:

1. Fluid pressure from the master cylinder spikes quickly.
2. The high-pressure fluid overcomes the spring pressure and moves the piston.
3. The piston closes the valve, restricting flow to the rear brakes.

When the pedal is released and the pressure is reduced, the spring returns the piston and valve to the open position.

Notes

• Fixed Proportioning Valves are available for stock and mildly modified vehicles.
• For performance vehicles, Adjustable Proportioning Valves are available.
  • These valves use a knob or lever to fine-tune the Brake Bias for various conditions.
• Height-sensing proportioning valves were used in some older vehicles.
  • They used a lever mounted between the chassis and rear axle.
  • During braking, weight transfers up and forward.
  • This moves the lever and activates the valve.
  • These valves are relatively rare today.
• Modern vehicles use electronic brake proportioning (EBP), instead of a mechanical valve.
  • EBP monitors input from the wheel speed sensors.
  • Then, it uses the ABS system to prevent wheel lock-up.