I have some experience with that, both 81 wings i have owned had brakes that were sticking. You can pull the calipers, tear them down, clean them well and replace the rubber parts. Not a bad job really, kinda messy and dont get any Brake Fluid on the paint. The kits come with the caliper seal, caliper dust cover, the accordian dust covers and the bleeder screw cap. if all the outside rubber is in great condition you can just purchase the caliper seal at the dealer. 7 bucks and some change each, versus27-30 bucks each for the kits. Maybe someone can give you a link to some cheaper kits.If I gothrough all of that, I will re-do the master cylinders also, as they are usually going to beas dirty. Then bleed the brakes really well, making sure you crack all the banjo fittings and then when you find out none of that has made your brake levers firm, tie the front one back with a piece of rope or zip tie, turn the handlebar to the far left and let it sit overnight. Weigh down the foot lever at the same time. That will let any trapped air rise into the master cylinders.
My 82 is having the same problem. Don't think the fluid was been ever changed. I flushed the rear system first to see what would happen. The rear is working fine. I flushed the front last weekend. We will see. If the return path to the reservoir is blocked (why the piston won't retract) then it could be master cylinder rebuild time or at least a disassembly and a good cleaning. I took the brakes off the bike and worked the calipers back and forth attempting to clear the blockage. I know, that is probably a half-ass way of doing something. But as I understand from many who frequent this forum, if the pistons don't leak, take the path of least resistance. Hope the flush does it.