How can just changing brake pads cause a low pedal?

[Jerry Bransford]

Changed the pads in my wife's JGC Saturday (biggest PITA pad-change I've ever done) and the brake pedal went to the floor when I started the engine and stepped on the pedal. The stopping distance is also now a bit longer unless I press harder than before on the brake pedal.

Brake fluid overflowed the reservoir while pushing the pads apart to place the caliper assembly back over the rotor, as it often will. Nothing unusual there. But after test driving the Jeep, the fluid level was still up near the top as if it hadn't lost any brake fluid at all. I'd have to guess a lot more brake fluid overflowed than there is now room for in the reservoir, which indicates it's full for all intents and purposes.

Ok, I plan to get the brake lines bled & power flushed tomorrow since that's the only thing I can think of, that air must have gotten into the lines somehow. But how? The brake lines aren't leaking and I didn't disconnect any of them.

Big mystery for a guy who is just competent enough to usually be able to change brake pads/rotors without too much thought needed.

Any ideas? Thanks!