Metering valves are used in rear drive disc/drum vehicles to hold pressure off of the front discs until the hydraulic system overcomes the return springs on the rear drum brakes. The metering valve is usually a 15 psi valve. The purpose of the valve is to prevent front disc lockup on ice and snow in light braking situations.

Proportioning valves are most prevalent in a vehicle with disc/drum combos, but are used in disc/disc vehicles in certain applications. Proportioning valves hold pressure off of the rear drums in a panic situation to prevent rear wheel lockup. Adjustable prop valves are available from most of the aftermarket hot rod brake suppliers. An OE prop valve will not work well in our hot rods because they are engineered for a specifi c vehicle. ABS has all but eliminated prop valves in current OE applications.

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.

I hope this clears up any misconceptions about hydraulic brake valves. As always, you can find me on the BBS most any evening if you have any questions.