90 Mustang 5.0 brakes-no indication if no pressure to rear brakes

I have a 90 5.0 Mustang. I apprently have been driving it two years with no rear brakes. I have changed the shoes, drums, and wheel cylinders and bled the system. I suspected something was wrong in wet weather braking and then when I was getting the emmission tested wtih the rear wheels on the dyno, the tech said I had no rear brakes. So after more work, I put the car on stands and verified this. The brake light only comes on for a loss of fluid or for parking brake set. There is no lost pressure sensing to a warning light from the master cylinder or the proportioning valve. In my case, I am sure the master cylinder is bypassing fluid at the rear brake piston. Does anyone know of a retrofit. I have checked part numbers through 93, and they are the same. The 94 (new body style) has a more simple master cylinder with just one port for the front brakes, but I don't know if it uses a different proportioning valve with a sensor and I don't know if a 94 master cylinder would fit.