1994 Ford Explorer leak between master cylinder and power b

Seperate the master from the booster, if the master is loosing fluid at the rear seal, replace it, now start the vehicle, have someone slowly press the brake pedal, any air escaping from the booster seal? If yes replace it. bench bleed the new master then install it, bleed at the lines first then the wheels if needed.
Do not allow the master to run dry while bleeding! Use DOT3 brake fluid only!