Brakes at witt's end

Brake problem: Even though I’ve had 3 master cylinders in my '02 Isuzu in the last 2 weeks, my brake pedal is still soft. It’s been bled several times, and the brake is still soft. Why? The brake booster has also been looked at, but doesn’t seem to be the problem. What should I do?

Before installing the master cylinder was it bench bleed? If not there is still air in the m/cylinder.

Check/replace brake hoses