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What is a metering valve?

A metering valve (sometimes also called a hold-off valve) is designed to engage the rear brakes just before the front brakes on cars equipped with front disk brakes and rear drum brakes. It does this using a combination of springs, seals, and pistons to restrict fluid flow to the front brakes until a relatively low fixed threshold pressure is reached. Forcing the rear brakes to engage prior to the front brakes is supposed to enhance braking stability.

Metering valves can be used as stand-alone valves or as part of a larger combination valve. They seem to be more common on cars built in the early 1970s. The metering valve image below can be found in the 1970 Chevrolet chassis service manual.