While running an errand this morning, my wife heard a terrible sound from the rear passenger tire on her van. She said she thought the wheel was falling off. She brought the van home where I pulled the rear tire and brake caliper and had a look. The inboard brake pad on the rear passenger tire was worn down completely and had worked itself out of the seating / guide slides. Her van only has 47k miles and these are original brake pads from when we bought the van new off the lot. The other brake pad from the same caliper is about half worn, as were both pads on the rear driver wheel. I replaced all of the rear pads today, and inspected the fronts. The fronts were all worn evenly and have about 40-50% remaining, so I did not yet replace them.
I am concerned about how a single pad in a caliper can wear completely down like this while the opposite pad does not. The pad that caused the problem is the pad that the piston pushes on.
Any ideas? Is there something else I should be checking for or replacing that I didn't notice? Upon seeing the pad, I thought I would have to replace the rotor also, but the rotor doesn't seem obviously badly scored or warped, so I'm thinking I'll just leave it.
Right now my best guess is that the worn pad was defective and a big chunk of it disintegrated suddenly at some point. All other theories I can think of involving something sticking in the caliper bring me back around to "if the caliper wasn't moving correctly, why is the mating pad worn about halfway instead of not worn at all or worn all the way down?"
Photos below show all 4 rear pads. The pads with the springs on top are the inboard pads. Ignore the weird circle on the bad pad -- that is from the C clamp I used to try to compress the piston. I used the bad pad as a load spreader.