I think that my first investigation would be to determine if the mechanical parts of the master cyl./booster/pedal package are correct. With the engine off, i.e., no boost from vacuum, is resistance from the M/C felt with less than 1/4 inch of pedal travel (free play)? If yes, do you then still experience a long pedal? If, however, there is a free pedal travel of over an 1 1/2 inch, there is a likely mismatch of push rod/booster/master cyl.

If the free travel of the pedal is normal, try bleeding just the M/C by alternately loosening the lines at the M/C and stroking the pedal as in manual bleeding (no power bleeder). It is possible that air is trapped between the two pistons in the M/C. (The booster push rod moves the rear piston directly, but the front piston is moved hydraulicly via fluid captured between the pistons.)

There is also the possibility that one of the cups on the M/C pistons will not hold any pressure, and that fluid is bypassing internally and returning to the reservoir.