The rear calipers require a special technique to "reset" them. They have to be turned clockwise while putting even pressure on them. The caliper pison should have a score where a tool is made to fit in to help aid in turning the piston. Most part suppliers have aftermarket "rear brake piston kits," with various diffent tools to help this process. The reason the calipers where designed like this is so that the e-mergency brake can be fitted onto the caliper itself. If both of your inner pads are wearing sooner than the outer by a large noticable amount, you may have a e-brake hanging up, or to tight of a cable. Once you have the rear brakes done, keep the wheels off the ground and spin them by hand, if they seem to have a drag to them possibly the e-brake might be an issue. Also, if you've used force earnlier on the calipers with a clamp, this may have bound them inside, and might take some force to turn them, if it seems extremely hard on one side to retract them now, than the other; one might have taken damage. Under normal driving condiitons the front brakes do tend to wear sooner, on late model vehicle's the braking ration is fairly equal and the front and rear tend to wear somewhat evenly, so if yours has a large difference and you've never done front brakes possibly the e-brake is wearing them, I would check it thouroughly.

-Rip