I just replaced my front disk brakes with Baer disk brakes. I replaced the Master Cylinder, and brake lines as well. I bench bled the Master Cyl. then installed the new lines. I had the front end up on jack stands with tires off, so I bled the fronts first, passenger, then driver side.
The fronts bled fine,but when I tried the back drums, nothing came out. I tried both sides, only a very small dribble once, then nothing. I even took the valve out completely to see if it was restricting somehow. I know I should have done the rear first, but could my doing the fronts first, somehow bias the Master Cylinder to where it does not feed the back brakes now?
I loosened the fitting at the diverter block as well, still got nothing dribbling out.
Any ideas?

I'll take a stab at it till someone else chimes in. If you got acess to air could you blow out the prop.valve to rear brakes?Maybe some junk got in the line.My 2 CENTS .

I had the same problem after replacing the entire braking system. Well, almost the entire braking system. The last part of the puzzle was the proportioning valve. I had one get stuck on an 83 Grand Prix I owned while trying to bleed the brakes. I took a shot and ordered one from OPG. It fit perfect, in spite of being an aftermarket part. I now stop on a dime.

Thanks Guys,
I researched some more and also came to the same conclusion, the prop valve. I first bled the line from the MC to rear brakes, nothing came out. I then took the MC off and had my son slowly puch the brake pedal to make sure the booster was not the problem, that was good. I put the MC back on, slowly tightening the nuts while my son pushed the brake pedal again. I watched for binding , etc. Once all tight, I was able to get fluid from the MC to rear line to Prop. valve.
I then ran a test light to the prop valve and it came on, indicating problem. I opened up the rear feed and nothing came out. I used a small screw driver as suggested in the archives, and pushed the plunger. It would only go about half an inch, then release back to original position, blocking the rears.
I also tried the method of opening the fronts and letting it bias itself, but that didn't work either.
So now before I order one, I'm going to take off the Prop valve and see if I can clean and reset the plunger to it's mid position. If that works, I'll put it back on and try again.
Thanks for the help, I appreciate it.

Just did the conversion and this worked for me. The new combination valve must have a big pocket in it?? Anyway I removed the single line that feeds the rears and used my thumb to cover the port as my wife applied pressure, being sure to cover the port on the upstroke of the brake pedal with my thumb. After about 5 strokes we had fluid. I reattached the single rear feed line and bled the rears and we were good to go.