LOL, MAKE sure your Vacuflush is ON before flushing!

[km1125]

Source of the post I have another possibility. It's like an implosion, first it collapses inward then outward as pressure equalizes. The vacuum causes an inrush of air that slightly pressurizes the plumbing if you let go of the pedal right away. This usually only happens on the last flush after the pump has been turned off. Because the pump is off, in a tight system, that pressure has nowhere to go. When you open the ball, that pressure is released to equalize and can blow out any water that's in the bowl on top of the ball. I think a few conditions have to come together for it to happen but I've had it happen many times in a lot of boats though not enough to get it all over me but Tony is a special guy

But with the duckbills both 'facing' towards the waste tank, ANY pressure should be relieved to the waste tank and out the vent.

[Cooler]

Now I am really glad mine is electric! Holy crap! er

[tonyiiiafl]

No $hit!

[Viper]

Source of the post

Source of the post I have another possibility. It's like an implosion, first it collapses inward then outward as pressure equalizes. The vacuum causes an inrush of air that slightly pressurizes the plumbing if you let go of the pedal right away. This usually only happens on the last flush after the pump has been turned off. Because the pump is off, in a tight system, that pressure has nowhere to go. When you open the ball, that pressure is released to equalize and can blow out any water that's in the bowl on top of the ball. I think a few conditions have to come together for it to happen but I've had it happen many times in a lot of boats though not enough to get it all over me but Tony is a special guy

But with the duckbills both 'facing' towards the waste tank, ANY pressure should be relieved to the waste tank and out the vent.

Yes I'm aware of that but my thought was if the bellows stopped in the full down position that it would block both the inlet and outlet ports as the ports aren't at the very bottom of the housing. The bottom of the original pump bellows was notched and may leave access to atmosphere even when fully extended but the newer bellows are flat bottoms and may not. I'd have to explore the amount of travel and whether the newer bellows clears the ports completely and is air tight at that point.