No rear brake pressure

[not2oldchevy]

AD with vette front and 8.8rear.with no pressure on rear brakes. New master cylinder, bench bled, new booster,new proportioning valve,all lines and hoses new. Bled 10 gallons of fluid[feels like] at every bleeder and line good front brakes no rear pressure. Any suggestions???

[G&R's57GMC]

Sounds like the proportioning valve to me. Bypass it and see what happens.

[not2oldchevy]

Checked proportioning valve it's new and the switch doesn't go to ground.

[OrrieG]

Is the MC below the floor, might need rear residual valve. Also, do the lines have any place where they could trap air?? I installed a coiled factory line from the 74 donor on mine vertical instead of horizontal and could not get the rears to work (lots of fluid but not pressure) because air was trapped in the coils. I turned them horizontal and fixed it right up. Are the shoes adjusted right? Too loose and you run out of pedal before the wheel cylinders are fully expanded.

Seems to be lots of brake threads lately....

[not2oldchevy]

MC is on firewall, no coils in line and shoes adjusted for slight drag.

[OrrieG]

Thinking out loud here, does it build pressure to the back when you pump them (just no pressure on the first pump)? Does the pedal allow the MC rod to fully retract when released? Are the rear wheel cylinders sealing?

You are getting fluid thru the lines to the rear so no blockage. Assuming you bleed them without letting air back into the lines push on the pedal should equal shoe movement. If there is still just a little residual air it may take time to work it out. When I put my truck on the road the first time after a complete change it took a month of driving, bleeding, driving, bleeding before I got it all out. If pedal pressure comes up after pumping it usually indicates air, which compresses. If you are getting pressure to the wheel cylinders but they are not moving two things could be happening. First wheel cylinder is frozen or scored so the seal can't move freely. Second is the seal is bad and the pressure just pushes fluid past the seal without moving the part that moves the shoe.

That is my thought process for figuing it out if it is a line or wheel cylinder problem. Since it is both rear brakes and not one, and odds of both wheel cylinders being bad is low, maybe it is something in the proportioning valve that keeps the fluid from moving but does not show it displaced.

I do not know enough about the inner workings of them to say what it could be. Like was suggested, I would plumb the rear line directly and see if it works. If the valve came as part of a front/rear kit it should have been selected to work with the different parts. If not, maybe there is something in the inner workings that is not compatible with the fluid volumes (line size or wheel cylinder diameter/volume), travel distance or ????

I hate brakes that do not work. Usually I have just kept messing with them and then they magically fix themselves.