I rebuilt my front calipers because I had one piston sticking on the right side. I was expecting a lot of goop when I poped out the pistons. Not so at all, it looked quite clean. Every thing moves good now. Wierd!
If you are using the appearance of the wear on the pads to conclude a piston was sticking, I think that would be a mistake.
Pads will wear unevenly because of the turning forces while you apply the brakes. This is especially more pronounced with smaller pads when compared to larger more expansive pads with greater braking surfaces.
The pads will wear more on the leading edge, and less on the trailing edge. This wear pattern will become more evident as the pads wear down.
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