1997 Buick Riviera brakes

The rear calipers have the parking brake built in so the pistons cannot be just pushed back in. they have to be rotated and threaded back in. They make special tools for that. If there is a lip on the rotor, you may have to pry the caliper off with a big screwdriver. It shouldn't be a problem if the rotor is smooth. Here's what a couple of the tools look like.