I took mine off and used some WD40 and a lever to move the piston, eventually freed itself up, might be worth a try before spending money on getting it overhauled?
Take calliper off
Take pads out
Press brake a little so the piston comes out a bit
Spray plenty WD40 onto piston(s)
Use lever to push it back in (you might have to loosen the brake reservoir cap)
Repeat until it frees up.