[DebNJim B]

I hear what Carl is saying and it makes sense to a point. But, when our grey tank gets full it will back up in the shower before going all the way up the vent pipe since water seeks it's own level. Same with the galley tank. And for that matter it would stand to reason the if the black tank is full, before it goes all the way up the vent it seems it would not flush down the toilet.

Or is there something here I'm not seeing? In all the tanks there is actually two points of pressure relief, the vent and the drains leading into the tanks.