Thread: Brake Nightmare

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RESIDUAL PRESSURE VALVES (RPV)
Residual pressure valves were used at the OE level until 1972. They were nothing more than a “duck billed” rubber valve in the outlet ports of the master cylinder. Their purpose was to slow down the violent surge backwards through the hydraulic system caused by the drum brake return springs. This surge would cause the cup lips inside of the wheel cylinders to distort and when the cup lips distorted, they would “take air”.

OE solved this problem by using “expander plates” attached to the ends of the spring inside of the wheel cylinder. These plates held the cup lips tightly against the inside of the cylinder bore and would not let the cup lips distort. Around mid ’73, Raybestos Manhattan in cooperation with Gibson Corp. invented a cone shaped spring with each end of the spring spirals cone shaped to fi t the inside of the cup lips. This eliminated the snap on plates that could come loose and score the inside of the wheel cylinder. The expander plates and the cone shaped spring ELIMINATED the use of RPVs at the OE level.

Some aftermarket brake suppliers would have you believe that the RPVs they sell will stop siphoning of the brake fl uid when the master cylinder is lower than the calipers and/or wheel cylinders. In order for a liquid to “siphon” one end of the tube or hose MUST be open to the atmosphere. In the case of a closed brake system, this is not possible.

One other thing needs to be said about RPVs!! If you buy a rebuilt master cylinder for your project, there is a possibility that it could be equipped with the duck billed valves (rpv) from the rebuilder. This is because many master cylinder applications use the same casting. If you install a rebuilt cylinder (which I strongly discourage) be sure to check the outlet port of the master cylinder on the disc side of the system. If you leave the rpv inside the port on a disc equipped car, it will overheat the caliper, pads, and rotors and could cause a premature lockup problem. If it was me I would get rid of them 2lb valves, I did that on mine and all my problems went away.