How does a brake proportioning valve work?

Updated April 17, 2017

In an automotive braking system, a brake proportioning valve reduces the hydraulic (fluid) pressure in the rear drum brakes when high pressure is required in the front disc breaks. A piston, spring and valve are used to balance, or proportion, braking pressure so the rear wheels do not lock up prematurely during baking.


A brake proportioning valve has an input from the master cylinder -- the hydraulic pump actuated by the brake pedal -- on one side and an output to the rear brakes on the other. Inside the valve, a spring-loaded stem slides back and forth inside the bore of an asymmetric piston to proportion the pressure.

Normal Braking

Under normal braking conditions, the proportioning valve does not restrict the flow of fluid. The spring prevents the stem from moving until a certain pressure, known as the knee point, is reached.

Heavy Braking

Sudden, heavy braking causes the pressure inside the system to increase dramatically, exceeding the knee point. The pressure causes the stem to move, pressing against the fluid restrictor and limiting the pressure to the rear brakes.

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About the Author

A full-time writer since 2006, David Dunning is a professional freelancer specializing in creative non-fiction. His work has appeared in "Golf Monthly," "Celtic Heritage," "Best of British" and numerous other magazines, as well as in the book "Defining Moments in History." Dunning has a Master of Science in computer science from the University of Kent.