Sorry to throw you a red herring guys. It was the hotel systems drain
sump. It has a vent at about the same level as the upper float
switch. I forgot that I had it on. Water flows into the vent and it
was the shower pump motor I heard running.

When the shower sump is off, my rube goldberg system works perfectly.

When the shower sump is on, it effectively becomes the bilge pump
keeping the system just below the level where the main bilge system
will activate. This isn't the worst thing in the world but won't be
an issue when I leave the boat and everything except the bilge pumps
are off. A short piece of tubing on the vent line will also take care
of it.

--

Roger Long