You might take a look at inspecting and possibly servicing the rear brake caliper, cleaning the external parts initially with clear water on a large towel (brake dust is hazardous, may contain metallic/asbestos bits). Then use pipe cleaner to clean around the exposed pistons as best you can, remove the pads and spray non-chlorinated Brakleen to wash the rest off (don't contaminate the pads if you plan to reuse them)...lightly sandpaper the linings and reassemble (pushing the pistons back into the bores). The key is keep everything clean, wash hands and area often.
http://www.hdtimeline.com/brakes_tires_and_wheels/9868-replacing_rear_brake_pads_softail.html