Strange behavior - Jabsco Head

[Capt.Don]

Looking for suggestions as to what would cause the following strange behavior with our Jabsco head. With the Y-valve set to discharge overboard, the toilet bowl would backflow with what appeared to be previously pumped out bowl contents. I thought this might be related to a blocked anti-siphon valve, so I disassembled and cleaned. That didn't solve the problem.

Only later, when we flipped the Y-valve to holding tank and pumped solid waste material through the system did the head resume normal operations. When set the Y-valve to overboard, the backflow did not occur. The head now appears to be working normally.

The only things I can think that caused this are (1) joker valve became inverted or folded in on itself, not making a seal or (2) the lower flapper valve became twisted or not set properly. Either may have occurred when the head was used (discharge) and the thru-hull was still closed causing back-pressure in the system. This back-pressure could have caused all sorts of problems!

Obviously something internal got wedged causing the backflow -or- the pump assembly is nearing a rebuild. Note, I just replaced the joker valve and the head is at least 2+ years old since any rebuild. Any thoughts as to the cause and whether I need to rebuild or wait. Thanks,

[caradow]

Is it a possibility that the hoses were inadvertently switched and you were really pumping into a full holding tank when you thought you were pumping overboard? Now the system works until your tank gets full again.

[hlev00]

With previously pumped effluent back in the bowl the joker has to be involved. As for why changing to the holding tank fixed it, that seems like a hard one to explain. Maybe there is less back pressure when pumping to the holding tank, and flushing through the holding tank dislodged whatever was keeping the joker from closing properly. If you usually pump overboard the might be scale build up. Another idea is that the y valve wasn't seated all the way over to the overboard discharge.

[Capt.Don]

Holding tank was empty, air vent recently cleaned, so I don't think it was involved. The Y-valve might have something to do with this, but I'm not sure why. While still acting strangely, after pumping dry, there still was pressure on the pump handle causing it to lift. Again, that's why I first thought anti-siphon was blocked, allowing siphon/backflow through the pump. Pumping solid materials through the system might have sorted out any dislodged or twisted rubber bits. Switching the Y-valve may be unrelated. Very strange!