conversion-single master cylinder to dual

Residual pressure valves aren't needed -- they were dropped from factory drum brake master cylinders (and the drum brake half of disc/drum master cylinders) after the 1972 model year; in 1973 wheel cylinders all got redesigned seals that do the same job as the RP valve (keep air out of the system -- that's all the RP valve was ever for, not to keep the drums from retracting too far).