Excessive brake pedal travel

I have a 2012 5.0 R-Spec with 87K miles. I got in my car one day and the pedal went to the floor after having been full top pedal with little travel. It has come back up since to about half pedal after one or two pumps but I wonder when / if that is going to fail. I've replaced the front and rear pads myself and had the master cylinder replaced by my mechanic. Any suggestions as to what is the issue or what to look for or do? Thanks