I don't know about Miatas, but on Hondas where the park brake is
involved with the rear disc brakes, you can't just push the piston
back into the caliper. You have to rotate it clockwise, literally
screwing it back in on the park brake adjuster. The piston has a +
shaped groove in it that allows you to get ahold of it and the pads
that go next to the piston have a projection that fits in that groove.

It's always good to have a shop manual for the car if you're going to
do that sort of work.

Don't forget to use a wire brush on any rust that's hiding under the
pad shims, and put a thin smear of grease there before reassembling.
That's a common cause of pads jamming and rear discs live a hard life
back there in the spray and sh.t coming up off the road.

Here's a site on Honda discs that may be similar:

http://www.tegger.com/hondafaq/Brakes.html#noise