[dsraven]

just thinking here, you did indicate the port closest to the mounting flange was connected to the rear brakes? did I get that right?
is the master cyl from a corvette with disc/drum? does it have a larger reservoir at the front-that being the one furthest from the booster? if equipped the large res should be for the disc circuit the smaller res for the drum circuit. if both reservoirs are the same size it could be a master from a disc/disc vehicle, connect the rears to the port closest to the booster. that way the rears apply first, controlled by the prop valve settings, then the front apply. that way the back end doesn't try to pass the front end when braking. you originally indicated there wasn't much pressure coming from the rear when pressure bleeding. possibly the prop valve is restricting? the master is faulty? spit balling here.