why does my brake pedal go to the floor when the car is running? on 2000 Ford Taurus

Driving my car one day and everything is fine, I park and let it sit for 2 days before driving again. But this time when I start the engine and try to shift to drive my break pedal goes all the way to the floor I can't figure out what is wrong. I checked for leaks, found none I added break fluid and still they don't work, what else could it be?

Asked by for the 2000 Ford Taurus
Internal leaking of the master cylinder. How empty was the reservoir , that you needed to add brake fluid?
It was low, does this mean I have to replace the master cylinder ?
NO , at this point , it should be properly diagnosed by a qualified person(s), you may have problems elsewhere in the system.