Soft Peddle

You can reach a situation (possibley defective residual valve?) where the pedal retracts fast enough that it sucks air in past the rear cup on "rebound."
X2. When you bleed the brakes, let the pedal up SLOWLY so it will draw in fresh fluid from the reservoir and not air around the rear seal. Also, I never push the pedal all the way to the floor. Half way on a new/reman unit is fine and only 1/4 pedal on a used master cylinder.