Question: you are sure the rear drum brakes are actually working with the RPV out? If they are then there must be some residual pressure there somewhere if you don't notice any nose dive or have a longer pedal travel than normal. How far does the pedal travel before you get rear brakes to engage? Have the lines to the rear been replaced to your knowledge? How about the switch assembly (right under the master cylinder).....has it been replaced or is it the original? Any possibility either may have some buildup in them? You bled something, was it the master or at the rear wheel cylinders? I suppose if there is some restriction and since the master is higher than the rear cylinders there might be enough head pressure to keep the cylinders out but I sure wouldn't want to rely on that.