K2 wrote:
The vented loop is behind the toilet, up at deck level. The y valve is
in the bilge 10" away from the thru hull.

Ok...NOW I get it. The y-valve isn't in the head where it belongs (and
where I thought it was).

Suggesting that you put it there is prob'ly a waste of time, but it
would sure solve a lot of problems....and also make it much easier to
switch between overboard and tank.

There is a Plan B that would eliminate the need for a loop or a valve
between the macerator and the thru-hull: put the macerator above the top
of the tank...or at least far enough above the waterline to keep sea
water out of the tank. It wouldn't be a vented loop, but it would
effectively create a loop that would do everything but break a siphon,
which would be unlikely. Macerators can be oriented in any direction,
even upside down, so you'd only need a surface on which to bolt it in place.

--
Peggie
----------
Peggie Hall
Specializing in marine sanitation since 1987
Author "Get Rid of Boat Odors - A Guide To Marine Sanitation Systems and
Other Sources of Aggravation and Odor"
http://shop.sailboatowners.com/books...ku=90&cat=1304