Eagleflyer,you just answered your own question.Soaking the piston and ring in Lacquer thinner will indeed soften this carbon based gunge and make cleaning the ring and piston groove much easier.In alot of cases,the softer elements of the build up will merely dissolve on their own into the lacquer but some harder portions made indeed need some physical agitation with an old tooth brush to help remove.Just be patient when your doing it and jiggle and slide the ring back and forth a bit at a time allowing the the thinner to better penetrate all around the ring,especially behind it.Jusy remember to use soft tools when cleaning to avoid any uneccessary scratches.All the best in your endeavours.