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.
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.
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.