1999 Chevy Camaro braking problems with the rear disc

1999 Chevy Camaro I've had braking problems with the rear disc brakes for about five years now. I've replaced the master cylinder, proportioning valve, rear calipers, rear pads, turned the rear rotors, replaced the rear center brake line and performed adjustments with bleeding the system each time. Colorado uses dynamometers for the emission tests which operate under the rear wheels. This is very difficult to test without adequate braking system for the rear wheels. Is there something I'm missing in the process, or has there been a change to fix this issue?