Not clear here. Has it gone to the floor since you replaced the master cylinder? If not, then the MC probably fixed it.
If it's still doing it:
Do you have ABS? I don't know squat about ABS, so issues with that are not addressed.
Have you made absolutely sure there's no leak anywhere, including rear slaves?
Generally speaking, if it goes intermittently to the floor and there's no leak, it's going to be a master cylinder issue. Don't forget the possibility of a defective new MC, especially if it's a rebuilt one. The booster when it fails will give you a hard pedal and high braking effort. It will not give you a dropping pedal. I am assuming here that you did a proper job of bleeding and the pedal is firm, braking even, etc. under normal conditions.
One other thing to check, though it usually only causes a soft pedal rather than a complete drop, is front wheel bearings. A wobbling disk can push the pads back when you turn, and give you an intermittent soft pedal.
