So here’s the culprit.
It’s not the donut in between the pipe and the flange; it’s the gasket in between the heat riser valve and the flange. I pulled the outer bolt off and let it hang loose by itself; the bolt and nut are bent and rusted, and there’s no trace of a gasket left there. So I ordered two copper-jacketed bolts and a new gasket from Super Scout, as well as a set of newly reprinted manuals. Finally. Everything should be here by the end of the week.
Oh, and speaking of the end of the week: I may be heading back to Chestertown to finish assembling the bumper.