a handy tip for freeing up sticking caliper pistons is to remove the caliper (keeping fluid lines attached of course) and using a G clamp (type used for clamping timber) push back the piston slowly to it's home position. It will feel stiff probably. Then wind out the G clamp about 5-10mm and have someone depress the brake pedal slowly and make sure they don't push the pedal more than half way to the floor (to prevent reversing the master cylinder seals). The caliper piston will push out to meet the loosened off G clamp. Now turn out the G-clamp another 5mm and depress the brake pedal again. Do not press out the calpier piston any further, it is near its furthest point of travel.
Using the G-clamp push the piston back in slowly to the home position. It should start to free up hopefully. Repeat the process again and it should start to take less effort the push the piston home.
If the piston movement does not improve then your caliper needs a new piston and O-ring, replacing these must be done with surgical room cleanliness and with competency. Soak the O-rings(seals) overnight in brake fluid in a sealed container before attempting to fit. I've replaced caliper piston seals before and you must operate like a neuro surgeon to do it right. Remember... make an arse of it(replacing seals) and you may suffer brake failure.
Hope this info helps.