I have had a lot of these issues with customer cars and there are a lot of things to check.
Just a few questions:
Power booster?
Is this a previous manual to power conversion? Having the wrong ratio can effect piston travel. Your master cylinder piston should travel at least 1" - 1.20".
When you bench bleed the master is it totally out of the car and you are pushing the piston by hand? If it is still on the car the piston may not be traveling far enough.
Are you noticing the lack of fluid at the rear calipers or at the master port?
Check flow at the master. If it is good keep going rearward. Go to prop valve, then after the valve, then at the rear diff hose. I have had a few bad rear hose preventing fluid flow.
Turn your prop all the way on. Knob all the way in or down, no threads showing. Adjust after you get the fluid flowing.
What calipers are you running in the rear.
How much air gap do you have between the pads and rotor?
Are the rear pads seated already?
Integral parking brake?
Things to look for.
When bench bleeding measure how far the piston travels.
With the master off, have a helper depress the brake pedal. Measure how much the push rod travels. Compare what you find.
If your pedal travel is less then you have a ratio issue.
Do you have an air gap between the push rod and master register? The push rod should not depress the master cylinder piston when mounted. That will cause a bunch of issues. Brake drag and the piston will be in partial travel and you can loose volume/flow there.
Hope that helps a bit.
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