If there is rust or calcium deposits, dump a quarter cup of muriatic acid (hardware store, Lowe's, farm and fleet, etc) in there with a quart of water...shake it around, dump away from the house, then rinse and do the same with bleach. Takes everything but grease out of there. Fumes are nasty so do this outside in well vented area.
This will also clean rust out of cement patio, clean out an old gas can, or tank, and will remove rust from any metal...tho you have to be careful about paint surfaces.