Hmm, I got my first sucker jam in the ZD-8915 since that first day.
I had recently put on the 1.2mm nozzle, instead of the 1.5mm, as the 1.5 was kind of oxidized away.
Additionally I had also started to replug the nozzle with tin, as people advise.
So firstly, I find with the 1.2mm it does not suck very well. And maybe in combination of adding a plug of tin when putting it away, after a few uses it jammed
Took quite some effort to clean. Completely packed with oxidized tin "sand", which never melts of course.
After cleaning I put back on the "bad" 1.5mm, and it worked better again.
My (anecdotal) conclusions: Keep using the big nozzle. Do not replug the tip with tin after use. Do run the rod through it after use, even when it is still open.
Also maybe this is a good motivation to add the delay valve, so it works better with a smaller nozzle.