Good pedal til car is running

Well i finally found time to work on it. I bought a new master cylinder just because it was not that expensive and i m gonna use the same style on my 71. At this point the new master is on. The pedal felt pretty good when i bench bled the master on the car. Everything felt good. Soon as i hooked it up to lines and bled it felt a bit softer. Then once i fired it.......Everything s the same. Super soft almost to the floor. No pedal basically. But the brakes work.