Probably a retaining spring that is old and worn out or has slipped out of place.
Probably your shoes and springs need replacing. Something you can do yourself.
You need to take the rear wheel off and keep it upright (oil in the gear box)
Take the nut off the right side (sprocket side) and use a rubber mallet or wooden block and tap out the shaft that goes through to the Final Reduction Gear box on the other side. DON'T mess up the threads on the end of the shaft (thus the rubber mallet or block of wood).
Once the shaft comes out the brake assembly should also come out. You can visually see if the springs are rusty or broken and see the condtion of the shoes and drum.
Shoes should have minimum of 2.0mm (.079") of pad lining.
Shoes are easy to take on and off, just fold the top one over like a piece of bread and the springs and shoes should come off. You can assemble springs on the new shoes and put them back on using the same method.
If you don't need to replace the shoes and springs, take the springs off and clean them with a wire brush. Lube (VERY lightly with a high temp grease) where the shoe ends meet the pivot bar that turns to expand the pads against the drum.
Clean the drum lining and pads with brake lining cleaner and fine or medium steel wool or emery paper. Wear a mask becuase some of those old brake pads may have asbestos in the.
Reassemble the wheel keeping it upright at all times so oil from the gear box doesn't run into the brake drum area.
You should be able to order parts at your local Honda MC dealer, www.partsfish.com or www.westernhillshonda.com