Q: hard brake pedal with engine off no pedal when started replaced all brakes rear clyinders and master cylinder no air in lines

From the description provided, my first step would be double checking the connections at the master cylinder as well as double check the flexible brake lines at each wheel. A small leak can duplicate the brake pedal falling to the floor. If you are 100% certain there is no air in the system, it is very possible that your F-250 may have a faulty brake master cylinder.

The brake system on the your 1987 Ford is pretty basic and if there are no leaks, then it is VERY likely the master cylinder is the problem. If you are performing the work yourself, you may want to try exchanging your brake master cylinder for a new one.

Be sure to properly bench bleed your brake master cylinder before installing it on to the brake booster as well. I would then bleed the entire brake system at each wheel just to be sure there is absolutely no air in the system.

Of course if you did not perform the work yourself, or if you feel like you’re stuck, then I would recommend enlisting the assistance of a certified technician. They will be able to quickly determine the reason for your brake pedal falling to the floor.