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I just replaced the front pads on wifes 91 Explorer with rear ABS. When compressing calipers so new pads would fit, I did it slowly, with a large c-clamp. Also adjusted the rear shoes - a bit too much slack. Everything worked fine Saturday and Sunday - took a 300 mile trip - but this morning, she tried to drive it to work, and came back complaining about having to pump the brakes up.
I checked everything out - no leaks, everything tight, fluid full in the master.
Sitting with engine running, first time you hit the brakes, pedal goes WAY down, but can then be pumped up with a couple hits. Same reaction driving it - once pumped up, it stops real well.
Sounds like the master cylinder went out. You'll need to replace it. Probably just a coincidence with the timing. Be sure to bleed the new one when you put it in before connecting the lines.