Originally Posted by mkguitar
pistons can seize in the bore if there is water in the fluid- rust.
is your brake system flushed of old fluid ?
The brake systems use a lot of different styles of "sliders" so it is nearly impossible to give a generic fix. You need to look at what guides the movement of the caliper, and then see what is making it not slide. It can be pins, dovetails, blocks, etc. Take the pads out, and you should be able to move the caliper back and forth easily by hand, until it hits the piston.
Lots of folks think that if you just beat the caliper with a mallet until it moves a bit back and forth, you have freed up the caliper, but you will find this is not the case. You need to find the bound up location, and file, sand, ream, derust, etc to get back to a free state, with high temp grease to help it last. Some styles still need yearly maintenance, others will go several years, but you do have to keep on top of it. Sitting in storage in a moist atmosphere is one of the worst cases you can have.
If you keep the calipers freed up, and the fluid changed regularly, calipers can last decades , but it does take some work.