b393capt wrote:

The current plumbing has a hose running up then down the wall to a thru
hull, in a loop,with an in-line filter and a diaphram pump at the top
of the loop. After I pump the water out of the head, even if I close
the thru hull, water runs back into the shower drain within a minute or
two. If I stick a cork in the bottom of the drain, the water gets
around it and forms a tiny pudle in the middle of the head anyhow.

I THINK there's a simple solution to your problem: pump longer.

The water that's running back into the sump is the water that's left in
the line between the sump and the top of the loop. Pumping longer should
move it over the top of the loop. If it doesn't--if dropping the water
level in the sump causes the pump to suck air that breaks its prime--a
one-way valve in the line just above the top of the sump may solve that
problems.

The thru-hull should always be closed except when actually dumping the
sump.

Btw...why would there be a filter in a sump drain line? Or do you mean a
strainer to trap hair etc?

--
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