Water gets in and swells the brake shoes and sometimes they fall off the backing plate too. Remove the nut, spring, and rod from the brake pivot arm. Use your favorite rust buster on the pivot shaft. Use a wood block and a hammer and beat the arm back and forth and in out, if you can till it loosens up. Use a propane torch if you need to. Try to figure out where center is on the pivot shaft so the pads are not pushed out against the drum. Is the axle out? Use the wood block and hammer and tap against the side of the stay bracket on the backing plate to break the shoes free of the drum. Patience; If you get frustrated, walk away and come back later.