brake pedal goes to the floor

Brakes worked fine, pedal just barely starting to get spongy. Mechanic said master starting to go bad. tried to replace master cylinder, new one had wrong size fittings. put old master back on and mechanic bled the brakes. the pedal went to the floor at this point. It did not go to floor before original master cylinder was removed and reinstalled. still stopped van. replaced with proper master and bled brakes. still went to floor. replaced master second time with another new one, bled brakes, pedal still went to floor. breaks stop van but go all the way to floor. no air in lines, no leaks anywhere. any ideas would be great. thanks