Brakes!!! new everything but combo valve

[lroy]

1971 k5 blazer

so here is the story... started tearing everything down and then a buddie called to have me come fish this weekend so i started to slap it all back on but i didnt wanna shortcut so i put new stuff on.

so put on new front rotors, bearings and races, studs, calipers and pads,

then on the rear i put on new cylinders and rear lines from the T

the slapped on a reman master cylinder from hi low.

so primed the MC then hooked up the lines, bleed the rears got new clean fluid, then the fronts and got clean fluid.

BUT... the system is bleeding off some where but i got no leaks.

with the truck off i pump the pedal and it builds pressure on the 2nd pump, nice and firm pedal. then i start the truck and the pedal goes in, so i know the power booster works.

symptoms..... i build up pedal pressure but then it bleeds off. cant even take the thing down the driveway bc i cant stop it. i have to pump like 3 or 4 times with it running to get any pressure. ive bleed the brakes and even pulled through a whole bottle of fluid just on that. im not loosin fluid anywhere, the resivor stays the same. my question is how in the heck does that combo valve work and do they ever fail? i have read that they can be adjusted back to centered, but it that my issue? it not like the rear brakes lock up, i just have no brakes period. and the light switch is on showing a difference in pressure. any ideas? i was hoping to roll down to the coast tomorrow and now here i am scratchin my head.