some one please explain to me how a residual pressure valve works...Im trying to understand where these things need to be used. thanks
Ryan

residual pressure valve is used in all drum brake applications, usally built into the master cylinder. After brakes have been released a slight amount of pressure remains in the brake lines, (not enough to overcome the tension of the retun springs) this hydraulic pressure remains in the system so that the seals in the wheel cylinders do not collapse, and there for start to leak.
there for you cannot use a drum master cylinder on a car with disc brakes, will cause the pads to drag, or vise versa disc brake MC on a drum brake car.

Proportioning valve used on drum / disc applications limits amount of hydraulic pressure to the rear brakes, prevents early lock up of the rear brakes, On 68 SS models I believe it is found under the drivers seat area, on 69 & newer models believe it is incorporated into the metering block under the master cylinder. (correction if I'm wrong are welcomed)