If you grab a hold of your rear brake caliper, can you wiggle it back and forth? Just noticed mine moves about 1/8 of an inch. The 2 bolts are tight that hold the caliper but the diameter of them is smaller than the holes in the caliper which they go through allowing the movement. Anyone know of this or can you check your bike?
Unless I'm missing something, the caliper on the rear is the older style sliding caliper with two pistons on one side only, versus the four piston calipers used on the front. The sliding calipers should exhibit movement, and the two bolts you mentioned are the pins that allow the two halves to slide relative to each other. The attachment is an exploded view of the sliding caliper.
Jerry
