Check the pins that hold the caliper to the caliper bracket.  The caliper
should move easily in the bracket.  If it does not, the pins are probably
rusted in place.  Replace the pins and rubber boots for the pins.  Clean the
holes for the pins thoughroughly, removing all rust and dirt.  Be sure to
liberally coat the new pins with silicone grease or anti-sieze paste to keep
them from sticking.

YMMV

AmmonMan