drum brake proportioning valve
First, I do not know if the brakes where better before ...................... The valve could cause a pedal to go to the floor (not like there is no pressure but you press real hard on the pedal and it looks like its near the end of the pedal when you stop........................ The mechanic that worked on the car said the back brakes were brand new and the front were in good condition.
I think in that case you are chasing your tail
IF the pedal has "excessive travel" BUT is HARD at the end of travel, then you have shoes out of adjustment
IF you have a hard pedal, and poor brakes, you probably have a drum/ shoe problem
Glazed heat damaged shoes
Drums turned too large, shoes don't fit, and have been glazed in the center of the lining.
Grossly improper lining quality
If you don't have (and need) residual valves, the ONLY thing the lack of them will cause is a deteriorating, mushy pedal over some period of time, that is, you will bleed it, and have a nice hard pedal, and after some amount of actuations, the pedal will start to get more and more mushy.
Another possibility, is, have you driven it enough to determine that ALL FOUR wheels are braking? Maybe you have the front piston in the master bottomed out, and are only operating off the rear piston.
The changing of the SAME BORE "single" to a tandem master should not have changed anything as applies to brake effort on an otherwise properly working system