I have used mercury outboards power tune and it really dissolves carbon but you need to let it stand for 4-6 hrs minimum.Keep spraying alittle more on.Once you break it free I would use a dry lubricant like powdered graphite in the threads or even moly powder.It really is a stupid set up that on-off brake.Cant the factory tell a owner how to break it free? I wouldnt be afraid to warm it up with a propane torch to see if it will let go.I wouldnt heat it red hot or anything but hot enough so you wouldnt want to touch the thing with your hands.Use a rag so the pliers wont destroy your brake.