Rear brakes lock up, front disc not stopping well

My '67 Notchback has never stopped the way it should. I'm running Kelsey-Hayes discs up front and 10" rear drums. I'm using all the factory setup from a '68 Valiant including power disc master cylinder, combination valve and all the proper brakes lines. Yesterday I took apart the front brakes, cleaned all the bores, pistons, rubber etc. Nothing looked really bad, pistons were not hung up and went back in easily. I have also changed all rubber lines with new ones and even went with smaller slave cylinders at the rear. They still lock up in the rear and the car does not stop well. Where do I go from here to solve this problem?