As a preventative maintenance tip:

Each time you remove the tire/wheel, take a large "channel lock" plier and squeeze the pistons back into their bores so they get re-wetted with brake fluid.

Just hold the pliers to the outside of the caliper and the inside of the "ear" of each brake pas and squeeze.

The piston won't go completely in, but pretty far. You'll keep things working fine for a long time.
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1989 740Ti 1986 240DL 1984 244GL

I recently had to do the same on my 760. Too late on a Saturday to get rebuilts. I removed the calipers, carefully pulled off the protective boots and sprayed the piston with PB. I then put the caliper in a big vice to push the piston in a little. If you have compressed air, you can work the piston back and forth a few times. Be sure to put a block of wood inside the caliper to prevent the piston from popping all the way out. If the boot is shot, you might want to get new ones to keep moisture out. My mechanic claims he uses this trick regularly and gets another 50K mi out of old claipers.

Skip
'93 850GLT
'89 765T
'83 242TI Flathood

Remove the pins. Eg. use a hammer and punch/phillips screwdriver. Check first to see if you have small 'cotter' pins holding the pins in place. (They're not actually cotter pins but I forget the name.)

Spray the pistons with wd 40 or like, to try to wash out any dirt/rust preventing them from moving inward. Chances are your rubber dust boots are done in.

Use vise grips or similar to try to squeeze pads toward caliper body. Ie. pushing piston in. Depress brake pedal. work both pistons in and out until they're free and pads can be removed.

The parking brake is a set of more conventional looking shoes inside disc/drum rear brake rotor. I think that you are referring to the pads stuck inside the caliper. If you can't pry them loose, try dosing the suspected tight spots with PBlaster and let them sit a while. If you have succeeded in removing the rotor and have gotten into the parking brake shoes, I'd recommend the same process. The trouble spots in the parking brake mechanism are the cables themselves and the cams that actuate the shoes.