Replace the master cylinder and hope it didn't take out the diaphragm in the booster by leaking back. It's going to the floor after filling because air is now in the system or the piston can't hold the pressure or both.

NOTE: Make sure you bench bleed the M/C BEFORE putting it on the truck. There should be instructions and the little kit to do it in with the replacement part.

Fasteddy's advice is occasionally sound...