I don't build up thick soft cakes as I wipe off the bowl after smoking, so over time a thin hard carbon layer forms and it's not been affected at all by a hot water rinse.
I always fold my pipe cleaner into a U shape and wipe my bowl with it after each smoke as well, however this cake that I’m talking about is about due for a reaming so I will give it another shot onece it’s reamed, however, I still don’t think the hot water is flushing out any of the tar and oils but I’m not so sure I want that to begin with. I’ll play around with it a bit more before I throw in the towel.
With estate pipes that need a DEEP cleaning I have always filled up a container with hot, soapy water and I let the pipe sit in that solution overnight. It makes it real easy to rid of the cake. This will swell the wood so refitting the stem is almost always a must once the stummel dries out.