Typically you want to press the brake caliper clyinders back into their bores in order to break any corrosion and re-lube the with brake fluid, so to speak.

I use a channel-lock type pliers on the brake pads "ears" and the outsides of the caliper to press things. (for PM sake, I do this everytime I remove a wheel no matter why. I also take a look at how evenly the pads have worn)

to be even more thorough, remove the pads, press the cylinders in even farther (they can go farther), clean, polish and lube the brake caliper hardware. Lube the backs and contact points of the pads and re-install.



If a cylinder is stuck, after removing the pad on the stuck cylinder side of the caliper, with the pad on the "good side" left in the caliper, lay a worn out pad in between the cylinders on one side of the caliper and the brake disc.

Stand on the brake pedal and see if this doesn't force the offending piston to move out a bit more, thus freeing it. Try it again, after a shot of PB Blaster or Kroil penetrant in the area behind the ruber dust cover (lift its inner diameter edge using a thing screwdriver tip, or remove the cover)

Now you can remove the rubber dust cover(s) and use some emory cloth on the exposed piston(s) to remove corrosion.

Just don't let the piston push out of the bore altogether.

I have even had some luck using a crow bar between the disc and the padded piston to press it back in from its stuck position.

The primary task it to get it moving again so you can try saving it.