You will need residual valves on front and rear brakes for a drum/drum system. The pressure is necessary to keep the wheel cylinder cups expanded to avoid air being drawn into the wheel cylinders. The main difference in disc/drum and drum/drum master cylinders is the size of the reservoir chambers. The reservoir for a disc brake caliper is bigger because the volume of the caliper bore is larger, requiring more reserve as the pads wear and the pistons move forward to take up the space. On a dual drum system, there's no need for the extra reservoir volume, but it doesn't hurt for it to be there. Racing parts places like Summit or JEGS should have the external residual pressure valves you need.
Jerry

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