I decided to leave this thing sit this week instead of driving it with no rear brakes. Today I was off work and decided to give Captain Hook's gravity bleed method a try.
Left master cap off and opened bleeder and....nothing came out. So I grabbed the wife and thought I would give the two person bleed method another try. I was getting good brake fluid flow out of both bleeders and seen no air bubbles come out. Did this for an hour.
I was just getting ready to give up when they decided to start working. My back brakes came back to life and the dash light went off. Glad that's over!

 

The centering valve shut off the rear hydraulic circuit and turned the light on. Once you get the air out, the system will build pressure equally front & rear. Then a good quick jab on the pedal will usually center the valve and turn the light off. You should probably do the gravity bleed now that the valve is centered, then hold firm pressure on the pedal and quickly open & close the bleeders. Only will take once or twice on each wheel.

 

Hi - had to replace my brake booster this weekend - it started hissing and then pedal got harder to push, motor got funky. Online videos pointed to the booster, which looked like a potentially easy job. But.... the master cylinder had to come off, I cross-threaded a nut into the distribution block putting it back together... had to rethread it. Then after it was back together I discovered a metal rod on the ground...turns out the pushrod had fallen out of the new booster before we put it on - luckily we just had to take off the master and insert this. But... even with it all back together, our attemps to get it bled before dark were inadequate and we had to abandon the project until tomorrow. Not a great day, but hoping a decent gravity bleed will get it working, and that the d-block doesn't leak and have to be replaced. Call me Murphy when it comes to car repairs!

 

Test Combination/Proportioning Valve
Use a test light by attaching a clip to a positive contact on the vehicle and touch the point of the tester to the electrical connection of the combination valve. If the the light does not come on, the valve system is operating correctly and no further testing is required.
If the light does come on, this indicates that the pressure differential valve is stuck in the .
Bleed the brake system to determine if the front or rear lines are blocked off. Set up one front wheel and one rear wheel for bleeding at the same time. Crack both bleeder screws and gently pump the pedal a few times.
The blocked side will trickle fluid out when the bleeder screw is cracked and the pedal pressed. An unblocked line will squirt fluid out the bleeder.
The lines that are clear must be left open and the blocked lines should have the bleeder screws tight to cause pressure to build up on that side. Be sure to use the standard bleeding procedures to prevent air from entering the system.
Slowly press the pedal with steady pressure a number of times until the light goes out; this will center the differential valve. You may also hear a pop come from the proportioning valve. This is the metering valve returning to its equalized position. When the light goes out, close the bleeder screw.