Tried a sonic washer for fuel nozzles, ended up with a container of cherry juice
Got a cheap ultrasonic cleaner off Amazon last year, thought I'd finally do those injector nozzles right. Filled it with the solvent from the parts house, dropped in one old nozzle, hit the button. Ten minutes later the thing smelled like a candy factory and the liquid turned bright red. Turns out the little O-rings and some carbon buildup mixed with that solution make some kind of reaction. Nozzle came out clean, but I had to dump the whole batch and rinse the tank three times. Now I just use the old wire and soak method, takes about 15 minutes longer but at least I don't feel like I'm brewing soda. Any of you guys ever had a cleaner turn into something weird like that?
Sounds like you just didn't rinse the tank well enough between uses or the solvent was already shot. I've run hundreds of nozzles through a cheap HF ultrasonic with straight parts cleaner and never once got cherry juice, so something else was going on. The O-rings probably weren't the problem either, those things don't dissolve in most solvents unless you grabbed something way too hot for rubber. My guess is you had old gas varnish in the nozzle that reacted with fresh solvent, and the red was just rust or dye from whatever was on there.
Solvent being shot doesn't turn red though, that's the part nobody's explaining. Old varnish mixing with fresh cleaner making cherry juice? Where does the bright red even come from then, because varnish is brown, not red. Rubber O-rings absolutely can break down in the wrong solvent, lots of cheap parts cleaners have ketones or aromatics that eat certain rubbers and they leach dye right into the bath. That's probably exactly what happened, the red was dye coming out of the seals, and your "hundreds of nozzles" just means you never dropped in one with the same seal material. Not rinsing the tank has nothing to do with it, a dirty tank makes the liquid dark, not candy red, so that theory falls apart too.