On my 97 F350 my brake pedal is very hadr to push down and does not return once applied. Under throttle the pedal is slowly pulled to the floor. I recently replaced my vacuum pump because it went bad now I am having this problem.
Sounds like you have serious Master cyllinder / Booster problems. Unplug the booster and see what happens. If the brakes work but are hard it is the booster. Other than that I would replace the Master ( assuming all is bled correctly and you have no obvious leaks and the fluid is good )
I pulled the vacuum line from the booster so i could get a load of wood a couple days ago and the pedal quit going to the floor. But as expected the brakes were very weak. I replaced the booster 4 years ago but have only driven the truck 5,000 miles since.