You need a caliper spreader that also turns the piston as it is compressed into the caliper since the rear calipers function as the park brake. The park brake actuator is threaded into the piston. When you apply the park brake the actuator turns and since the piston is keyed to the inner brake pad this causes the piston to extend rather than turn also and apply the brakes. The cube compressors are the cheapest. Just drive the piston into the caliper with a 3/8" extension and ratchet and push as you do it.
Sent from my Droid X. Typos probable.