Not sure what you mean by "comes back on", but once the pedal went to the floor, the master cylinder is likely damaged and needs to be replaced. Crud and corrosion build up in the bottoms of the bores where the pistons don't normally travel. When the pedal goes more than halfway down, as in when you're surprised by an unexpected leak, or more commonly when do-it-yourselfers pedal-bleed the system and push the pedal to the floor, the lip seals run over that corrosion and can be ripped. That results in a slowly sinking pedal that often doesn't show up for a few days.

Given the age, I'd recommend replacing the master cylinder, then see if there is still a previous problem. You can push the pedal all the way down when the master cylinder is less than about a year old, but professionals are in the habit of never pushing any brake pedal more than halfway down.

Answered by caradiodoc (expert)
17,277 answers provided