I'm posting this as possible help for anyone with similar issues to what I have experienced. 73 Duster with power front discs and rear drums from 74 Valiant donor car. The donor car was drivable and the brakes always seemed a little sensitive but usable. After swapping all parts to the Duster, including proportioning valve and rebuilt booster because original blew diaphragm, I had a very low pedal and the brakes would try to put you thru the windshield if you didn't brake like there was an egg between your foot and the pedal. Bled the brakes, including rebuilt master, and re-adjusted push-rod several times with no change even though pedal did not pump up, indicating no air in system. Pedal height was no different whether or not emergency brake was engaged. With the car up on jackstands, the front brakes would engage immediately, but it seemed that the rears would take a couple of pumps before fully engaging even with the shoes adjusted out tight to the drums. Out of desperation I basically ended up throwing parts at it, but the brakes work much better now. I still think that the pedal seems a little low, but you actually have a little feel to the pedal now. In hindsight, I think I would have just used a manual master and left the booster off as I don't know that it's really necessary to have for a lightweight a-body. The parts replaced were the following - New dust boots and seals in front calipers, rebuild kit for prop. valve, new wheel cylinders, shoes, and hardware on rear drums, all new hard lines. Not really sure that the hard lines were necessary, but now I know what I have. I realize I have written a book, but hopefully my experience might help out someone with similar problems. I can actually let my wife and son drive my car now that every stop is not a panic stop. I assumed while doing the work that I would find a bunch of crap in the prop. valve or elsewhere but I never really found a smoking gun.
