Doc/Commander, well done on your inspired choice of vehicle!!
I personally think they are drastically underated in the MV world, although not on this Forum!!
Doc, drop "SIRHC" a Private Message, he has a few NOS Brake cylinder rebuild kits at a very reasonable price, I have just bought one to rebuild my spare cylinder, it seems to be a problem with these vehicles that the brakes are a bit dodgy as they get old, but a thorough rebuild should sort them out, they arent difficult to do, just a little time consuming.
Sounds like the "cup" you cant get out is getting stuck on a rust lip, it is a fairly tight tolerance with the bore, carefully clean it, if it is still proving a bit tight, push the piston in, grease the bore and then apply some compressed air to the union at the other end, should blow out nicely (make sure something is in front to stop it going into orbit) then clean out thoroughly making sure no grease remains and have a check of the bore condition.