That is a Class A PITA job on an older car with the MC under the floorboards. I would look for a line restriction somewhere on the line going to the front - there is a T somewhere on this single reservoir system, might be at the MC itself. I'd start there. I would also look at the brake light switch - I believe it to be hydraulic vs the mechanicals of later. Is it restricting flow? Is there a kink in a steel line? A rubber front wheel cyl. line? Is the new MC any good - have the piston seals gone bad from long shelf life - rust? Is there a check valve in the MC? All under floor MC drum/drum have one as the wheel cyls are higher then the MC.