RE: Braze on muffler nipple?
I think you could braze it in (good thinking) but it might be too easy to melt the brass nipple in the process. Wouldn't hurt to give it a try. It doesn't seem likely that the muffler got so hot as to melt the solder though. The problem could be that once a muffler is used, it is very difficult to get all of the castor oil out of the pores on the metal. Try heating it with a torch and then use a wire wheel to clean it down to some bare metal. You could try some silver solder which has a higher melting point, or even JBWeld as an experiment. Getting back to the brazing, you could simply braze in a tiny steel tube and be good to go. You will get this solved.
Ernie Misner