Brake problems

Ok, we have a 1968 dodge charger that had factory drum breakes, it had disc breakes put on from a 1973-6 a body. It got every thing from calipers to master cylinder from it. Ever sense the conversion it has had trouble.
The back brakes lock up while you have almost no front brakes unless it is pumped multiple times. We have bled brakes multiple times, and changed calipers and hoses, still the same. I pulled the proportioning valve and it was full of gunk, could that be the problem? And if so is there anyway to rebuild it? If not any other suggestions?