brakes not braking

[joebuick]

Hey Guys,, I have a 80 K10 that I am restoring with my grandson and have come up on a puzzler. The brake pedal will get very hard at the bottom of its travel and does not stop the truck very well. A few close calls and we have parked the truck till we figure this out. I installed new rear wheel cylinders w/ new shoes. The front disc pads were both OK so didn't do anything there. We installed a new master cylinder and still no help. Since this truck had been parked for several years,,, could maybe the proportioning valve be plugged? I have seen flex brake lines collapse inside and lock up the brakes but these do not lock up but get VERY hard the last inch or so and danged if it don't seem like the truck speeds up,,!!! Any help on this??