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
1 answer 2 comments
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.