Hy 1pete1, what was the original application for the dual master cylinder you installed, was it for disc brakes or drum brakes or disc/drum? Residual pressure valves are built into master cylinders for drum brakes or the drum brake section of a disc/drum master cylinder. When we have a mixture of brake types (Disc and Drum) we usually require a proportioning valve to stop the drum side of the braking system from locking up under the increased pressure that the disc brake side of the system needs to perform properly. Master cylinder bore size is also important, I would not use a master cylinder with a bore smaller than the one that controlled the brakes on the truck that you got the rear axle from.