stuck brake pedal

the brake pedal on my 69 Ranchero sticks half way down and won't let the car move. I can lift the pedal with my toe and then drive until I hit the brakes, then the pedal stays down applying the brake. It has 4 wheel drums with power brakes. Master cylinder, power booster and the rest of the system is new (shoes, drums, wheel cylinders, even brake lines). The only old part is the proportioning valve, and there is no replacement available. I tried recentering the valve as instructed in the factory service manual, no change. Can't find anyone local who even has a clue. Help!!!!!

Your power booster may be defective or misadjusted. See what happens when you unplug the vacuum line from it.