It's also possible the master cylinder is just too small- - - -piston diameter, stroke, or both. If one push of the piston isn't sufficient to displace enough fluid to push all the shoes against the drums, all the bleeding in the world won't help. Try blocking off the line to one rear wheel temporarily and see if you can apply the brake firmly to one hub at a time. The original single line MC and hydrovac was probably there for a good reason.
Jerry
