Sorry about the length, but im trying to cover all the bases here....
Okay, so this has probably been covered, and i missed it in my searches, but heres the issue: I have no rear brakes. I have an 86 Firebird, originally equipped with a drum brake differential in the rear. I swapped to a 9 bolt from an 85 Trans Am with Disc Brakes. I changes the rear, thats all, not the maaster cylinder or the proportioning valve. Everything was good, and seemed to be working well. Last week, my brakes started feeling spongy. No leaks, but very little pedal, and the light comes on. So i figured it was the master cylinder, as there did not appear to be any air in the system when i tried to bleed the brakes. Anyway, i put on the master cylinder from my parts car, which was also a disc/drum car. Bled the hell out of it. No change, and still no rear brakes anymore. My fronts are working fine, but no rear brakes. What could have caused this, and what can i do to possibly fix it? Would the prop valve and master cylinder for 4 wheel disc brakes help? I would really like to fix this, cause im getting ready to go back to drums if i cant get it sorted out soon. Thanks ahead of time for any assistance at all.

 

Try re-bleeding those rear brakes. If you push on the pedal, and crack open a bleeder screw at the same time, fluid should squirt out. Consider also the possibility that your rubber brake lines are bad and it's leaking fluid onto the asphalt. If you swapped master cylinders and got it properly bled, there is nowhere for the fluid to go except into those rear pistons. If you get no pedal, then either the pistons are radically retracted away from the rotor faces or else there's air in the line.

so,
1) check to see how far the pads are from the rotor faces. They should be touching.
2) Try bleeding those rear brakes again. Use a buddy who pumps while you crack open and close the screw. The buddy applies moderate pressure on the pedal, you crack open the screw, he calls out when the pedal hits the floor ,you close the screw, and then he lifts his foot. Do it in that order. Flush about six ounces of fluid through and dont forget to top off the master cylinder every so often.

 

Thanks for the advice, but i've bled the brakes 3 times before now and im still not gaining anything. The pads look like they're supposed to, just touching, but they still arent doing anything.

 

All that's left is that combo valve, in that case. There's got to be air in there somewhere. Either that or the fluid's boiling due to a header pipe being in contact with a line. That's all I can think of.

 

thanks again