Try heat. The caliper is aluminum while the bleeder is steel and will expand at a lesser rate. If it does break, don't try to drill it out on the bike. Take it off and use a drill press and go slow. It is possible to helicoil, but take care not to damage the seat at the bottom of the hole. The calipers seem to still be plentiful for some reason, so it often is easier just to buy another and rebuild it with stainless pistons and new seals.