Sigood,
just because one of the pistons doesnt come out when the handle is pressed, doesnt necessarily mean that there is a problem - it just means that one of the pistons has less resistance than the other.
Hold the piston in that moves (use a clamp or your hand) and then press the handle. If the other piston (the one you suspect has stuck) still doesnt move then you have a problem.
If the piston does come out it should be easy enough to push back in with you thumb (dont let it come all the way out or youll have to bleed the brakes. If you cant push it in with your fingers then it needs cleaning up/ overhauling.
Note if you clamp the piston that appears ok, and use the brake lever I would still expect the (suspected stuck) piston to move (even if it was stuck) because that is one of the ways to get a stuck pisotn out.
The real test is being able to push the piston back in fairly freely.
And yes, they are easy enough to overhaul.
Steve