Couple of things to consider. If all drum system, you need a dual master that is about 1 in. Any larger and you get a hard pedal. Second, no new dual master comes with pressure residual valves. The original single pot master had one to keep a little pressure on the wheel cylinder cups and to keep the springs from fully retracting. Original dual masters from back in the day, and rebuilt masters from back in the day had these little duck billed pressure residual valves. One each for each port in a drum brake application, one in the rear port for a disc/drum set up.
If you look into the ports on an original master, you will see that the seat is made of brass. They were removable. The valve and spring were installed behind the brass seat. With todays new masters, the seat is cast iron machined directly into the casting. No way to retrofit a stock type duck bill residual valve. Rebuilders like Cardone stopped installing the residual valves a number of years ago but the Wagner rebuild kits still have them so if you had an original master with a removable brass seat, you could rebuild the master and install the valves.
For today's masters with the cast iron direct machine seats, you need to install a 10lb in line residual valve to compensate. Wilwood is one source for those valves.
So it may pay you to pull the master and see what you have size wise for the bore of your master. If its wrong, or the unit is all junked up, now is the time to replace it. Sort out the pressure residual valve issue, then the rod adjustment issue. From that point you at least know the major parts are correct.
