My 1987 Ford Pickup Truck 150 has 200,000 miles on it. My problem is when I apply the brakes the brake pedal feels like mush--and the pedal goes all the way to the floor the truck will stop for now. I have replaced the master cylinder and bled on 4 lines and there is no change. I can unplug the power booster and cover the hole with my finger and get pedal. The power booster is only 1 year old and I replaced it because of the above mentioned problem. I don't know if replacing the booster will help.
Did you bench bleed the master cylinder? Also you still could have air in the brake lines. First make sure all air is out of the master. Bench bleed if neccessary, then gravity bleed all four wheels till the fluid runs clear. If the master cylinder is mounted at an angle, you'll need to jack either the front or rear of the vehicle to make sure the master is level with the floor. While the wheels are gravity bleeding, lightly tap on the calipers to let any air that may be trapped in the calipers. After all is done have some one pump the pedal while you bleed the brakes. Do this seqence, right rear, left rear, right front, left front. Then bleed the lines at the master cylinder. If you still have a bad pedal then I would suspect a defective master cylinder. Sometimes new parts can be defective. Good luck I hope this info helps