Hy don stocker, a disc/drum braking system must use a dual master cylinder for a disc/drum system. If your system currently uses a single Hydrovac, another Hydrovac must be added for the second system. Disc brake systems do not have or require residual pressure valves, unless the master cylinder is mounted below the level of the disc brake calipers and then only a 2 p.s.i. residual pressure valve is required in order to stop drainback to the master cylinder, dual master cylinders for drum brakes have a residual pressure valve built into each system, hope that helps.
