Fronts bleed, but not the rears...

Oh, to beat a dead horse. I've read the various threads, but I'm still not having any luck getting brake fluid to the rear brakes. (Fronts bleed fine.)

The car sat for about 8 years, so I replaced the prop valve, rubber lines, rebuilt the calipers, and the drums - so the entire system was dry. I get the feeling there is something blocking fluid to the rears.

I've tried the following:
- Bench bleeding the MC (while still on the car / sitting level)
- Mity-vac at the rear with the "button" on the end of the prop valve both pushed in/out)
- Manually bleeding w/ brake pedal with prop button both in/out

The button on the front of the prop valve does not go in very far (like 2mm), but I've also read that pushing this button isn't necessary at all.

I did see a tool the screws into the top of the prop valve where the switch is located. This is supposed to open the valve, but again I've only seen it referenced once - so I'm not convinced.

Any thoughts?
Found this...

Looks like this little bleeding tool might be necessary in the place of the brake sensor. Haven't seen many references to this.
Two things that have prevented me from getting fluid to come out of the calipers/cylinders.

1. Old, swollen, broken down rubber lines. They can break down internally and look fine on the outside. This internal breakdown have prevented even the biggest man standing on the pedal from getting fluid out.

2) clogged bleed screw. This can be fixed by removing and cleaning or replacing.

Sent from my iPhone using Tapatalk
All the rubber lines are new and the bleeder screws have been thoroughly cleaned.

I think the prop valve bleeding tool (in the video) might be necessary, but it's odd that it is not reference any other threads. My guess is that most people aren't refilling a dry/empty system. There is still some fluid pressure in a filled system that is only being bled, and that is keeping the valve in place.
Back when I was having brake issues, I experienced the same issues you did. I began with the prop valve, thinking it'd fix my soft pedal. I replaced it then tried bleeding the brakes, and could only get fluid in the fronts but not the rears. tried it by vacuum bleeding, pressure bleeding, with Teflon tape without it, still nothing. Then I got rid of the powermaster brake system, put a manual brake kit on and stainless braided brake likes from Russell, and I got fluid on all 4 corners. I followed the bleeding instructions provided and got good results. I've read it could've been the master cylinder or the accumulator, but back when I was going through these issues I believe the price for those parts was more than the manual brake kit. It cleaned up the engine bay at the same time. The first drive I didn't care too much for the different feel, but now I love it and wish I had did it earlier.

hopefully some of this helped, these are just pieces of my experience and what I ended up doing with the car

Sent from my iPhone using Tapatalk
What type system on car?
It's the Power Master.

If I understand how the combo valve works, I think the issue is that there was no fluid pressure on the rear brake line causing it to close when I was bleeding the MC "by foot."

If I release the pressure (bleed out) the front calipers, that should release the pressure and allow the valve to re-open to the rear. Then I should be able to vacuum bleed the rears. This way I can avoid having to buy/fabricate the combo bleed tool in the video.

Once I bleed all four corners, I'll bleed the MC, and then bleed the corners one more time.
Quick update... unscrewed the line that feeds the rear brakes from the combo valve, and I actually got fluid flow. So the combo valve is not the issue.

Then I tried the driver's side rear brake - I got flow. Tried the passenger rear - again, no flow. Unscrewed the feeder line to the passenger rear, and got flow. So the issue is actually the wheel cylinder.

Removed the shoes, and applied a little percussive maintenance to the cylinder... now it flows. So something in the cylinder/line was blocking the flow. So I guess I'll be replacing the wheel cylinders now for good measure.