Take wheel off, take drum off, look for brake fluid, change cylinder. You can get overhaul kits, but it's probably easier to get a new cylinder. Check the adjuster while you are at it as they are notorious for seizing up, and when they do they are a right b45stard, worse than any other studs/nuts/bolts that I know of.
SEP field working, not spotted in 103,100 miles. Some photos on webshots, updated 25 Jan