First thing i do is check that its not the pads sticking on or the sliders or anything daft like that ive seen alot of cars with a brake binding problem that once the pads are removed and everything wire brushed and re-greased it can solve the problem if its the caliper itself you can remove the piston, clean any corrosion re-grease that then rebuild. Not sure if there is a caliper refurb guide on here.
Alot of people just replace with another caliper if its standard brakes as its only roughly £10 for one but they can all be taken part and refurbd.