I'm experiencing the same issue. We have a 2014 Rockwood Ultralite 5'er, and the John worked fine for 2 seasons. This year, I had to replace the flush valve due to a pin hole in it, and now the vacuum breaker is spitting. I notice that if you release the flush pedal slowly, it won't do it, but if you let up on the pedal quickly you get a real good spit.
I've been winterizing by blowing out the lines, and suspect a bit of water must have been left behind and caused these problems. Next year, I'm pumping in antifreeze for sure.
Does anyone have a contact number for Diane at Dometic?
Lou & Paula
2014 Rockwood 8289WS Ultra Lite w/Reese Revloution Pinbox.
2011 F250 Power Stroke Super Duty King Ranch Super Crew w/Reese 16k Fixed Hitch