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What is a proportioning valve?

A proportioning valve is used to reduce the amount of brake fluid pressure applied to the rear brakes. They are often (but not exclusively) found on cars equipped with front disk and rear drum brakes, and are not typically found on cars equipped with four-wheel drum brakes. The valve assembly contains a combination of seals, pistons, and springs that work together to reduce the pressure of fluid passing through the valve.

Proportioning valves can be used as stand-alone valves or as part of a larger combination valve. OEM valves are typically configured to provide a specific output pressure, but adjustable valves are commonly available in the aftermarket. They are often used when adding front disk brakes to cars originally equipped with four-wheel drums.