sounds like the proportioning valve went over center, probably when the fluid for the rear brakes went dry when the wheel cylinders started leaking. I've had luck in the past by not having the truck running, push the pedal a few times to get rid of any residual vacuum. The push the brake pedal hard and fast a couple times. Other wise you may have to take the rubber cover off the centering pin on the proportioning valve and then push the pin manually to center it.