Brake trouble

[ssfirefighter]

I have replaced my master cylinder x 3 now. I just bought a new, and not rebuilt unit. I drove it for two days with a little bit of spongy feeling, and was waiting to bleed them. On the way to work this morning I felt a shudder and started to pull over to see what it was, and noticed that I had no brakes. They felt really spongy, then the pedal went to the floor and would not return itself. I flipped it up with my foot, and pressed again. I got a little pressure, then back to the floor and no return again. Any suggestions?