To expand on jd's post, it sounds like: Both pads were removed, piston A pushed back, piston B pushed back but A crept forward.

The solution is to push one piston back, insert pad, push the other back, insert pad and pins.

Sometimes the sides of the pad have burs that need cleaned off so use a file to take the high spots off. If you really do need to thin the friction material, 40 grit sand paper works just fine on a flat surface