Stuff in brake fluid?

Mushy pedal after that much fluid bled through it makes me suspect contaminated brake fluid. Someone in the past may have goofed and put some oil in the master cylinder. I suggest changing the master cylinder, and rebuild the calipers and wheel cylinders. A real good clue of contamination is the rubber seal on the master cylinder cap. If it's deformed and enlarged where it doesn't fit right you have oil in your system. If that's the case for sure change your rubber hoses and proportioning valve also.
I've found this happens a lot when your oil is changed at a quick lube location. Those guys are rushed so much that sometimes they put the wrong fluid in the wrong places.