C20 Brake Problems

[gmsnowball]

70 CST20 dually front discs rear drums. Bought a speedway power brake booster, master cylinder and proportioning valve and can't seem to get them to work. First master cylinder was bad, would not bench bleed, they sent another one that also won't bleed or refill as it should. Is there a problem unique to C20 master cylinders? Front brakes work but rears are almost nonexistent. The chevy dealer can't seem to get the master cylinder to work. They tried master cylinders from other suppliers without sucess. Could the plunger be too short and not forcing enough fluid to the rear portion of the master cylinder? It seems like even the dealer can't find the problem. I have never had trouble with any brakes like this before.