arms and pins? i dont quite know what you're talking about. unless you're talking about the pins that the caliper slides on.

the calipers dont slide up like on the fronts. you need to remove both bolts that hold the caliper in. If you put the caliper in a strange position it will cause to caliper to compress because of the ebrake cable.

I would try to compress brake piston more, by twisting it clockwise. And make sure the ebrake is off. wheel should at least spin. It shouldn't be tight.