Best Of

We've also noticed that the grey tank seems to fill "too fast".  We have started to chock up the wheel opposite the grey drain as high as we can with our Lego blocks when dumping.  This seems to help some.  Residual water in the grey tank after dumping would obviously reduce our capacity. (It also drains faster).

A word on the sink Hepvo valve:  It stops opening properly to drain the sink if the back pressure on the valve matches or exceeds the head of water from your sink.  If the sink is nearly empty, this could be just a few inches of water.  And that back pressure need not be water, it could be compressed air.  That pressure wouldn't be enough to force water up into your shower.

So; a theory:  If the vent into the grey water tank was badly designed so that it doesn't end flush with the top of highest point of the grey water tank, but protrudes in like this:

Then when the tank is filled above the bottom of the vent pipe, pressure will rise (as the air cannot escape) and quickly stop the Hepvo sink valve from allowing more water in.

You can test this out:  When the sink starts to back up, just pour more water into the sink.  Does any of that water drain away?  If it does, then the tank isn't really full - you just have back pressure stopping your Hepvo valve from opening.  If it doesn't, your tank really is full and now you have a bigger mess on your hands!