I think you are mixing up a residual valve and proportioning valve. The residual valve holds a slight amount of pressure in the rear lines (13 psi?) to overcome the spring pressure that otherwise would delay engagement of the rear brake shoes. The proportioning valve will NOT hold pressure in the rear brake lines. It merely allocates fluid between front and rear brakes.
The residual valve is no more than a rubber seal looking piece which fits behind the seat in the master cylinder. If you pull off the brake line to the front reservoir, then thread a drywall screw part way into the seat, you can yank on the screw with pliers and remove the seat. Behind that is where this residual valve should be.
In my limited experience, most rebuilt master cylinders lack the residual valve. Don't know why, but they are simply missing. Once you install the residual valve, you can either install a new seat or take the old seat and use fine sandpaper 220 or finer, to take out the scratches and goobers made by the drywall screw. To install the seat, simply place it into the hole and press it LIGHTLY so that is alligned straight. Then, when you tighten down the flare nut on the brake line, it will seat the seat to the proper depth. Makes a nice tight joint.
For more on the difference between proportioning valves and residual valves, see the website for Master Power Brake Corp. They probably have the best technical discussion of these valves and their operation I have read in 10 years of the hobby.
Rebuilding a classic mustang is a reality check on your individual state of perfection.