Brake pedal to floor soooo very slowly?
Try to get the best quality master cylinder you can, preferably new (not remanufactured). RockAuto.com has a good selection I've bought a few off there with good results... Wagner is the brand that sticks in my mind but I think those are being discontinued unfortunately.
I once got a reman'd master cylinder from O'Reilly for my '70 Duster (manual front discs) and only a couple months after installing it the pedal would go straight to the floor if you didn't press on it fast enough. Typical for a rebuilt unit with a honed bore, the seals might not fit properly because the bore is just slightly too big.