Chevrolet Repair: 1989 Chevy K2500, rear drums, wheel cylinders

Question
I have a 1989 Chevy K2500 4X4 5.7L with front disc brakes and rear drums. I do not have hydro boost. The question I have is that I can build up pressure in my brake system, but as soon as I back up, the pedal drops straight to the floor. I have replaced both rear wheel cylinders, both front calipers, the vacuum assist system, the master cylinder, and the anti-lock control system. I am lost and I was just wondering if anyone has had this problem before. Thank you in advance for your time and consideration.

Answer
Hi Dennis, couple things come to mind, did you by any chance install the calipers on the wrong sides.. bleeder must point up if they point down change them side to side, or did you by any chance invert the brake lines on the master, front brakes have to come on first so front brake line should be connected to the chamber closest to the vac booster.if not them then check the purportioning valve, make sure pin is centred... hope this helps