Mine will do this on occasion too. Jabsco
Quiet Flush. Mine goes directly into a holding tank
though. The amount of back flow I get is the same amount that happens to be the capacity in the discharge line, that passes back thru the joker valve. Luckily I'll never have an overflow since it's a holding tank
and limited to the capacity of that discharge hose. I replaced the valve last week and it did it again. I'm finding the wife doesn't flush long enough to make sure there was no TP caught in the joker valve holding it open. If flushed clear, then just enough water
to fill the small opening in bottom of bowl, then flushed again, it is fine.
Ordered a new higher volume fill pump to install this weekend to make sure I get a good amount of water
to push all contents past the joker valve. Current
pump only supplies 1 gal/min. Jabsco recommends a 3.3 gal/min pump.