Sounds like your pump needs a rebuild kit, also a vented loop in the hose
between the top of the pump and the top of the bowl. I think the lake is
slowly filling your tank all the time.

I agree with your diagnosis, but only part of your recommended
cure....neither rebuilding nor replacing the joker valve can solve every
problem...in fact, actually doesn't solve 99.999% of problems.

Rebuilding the toilet (replacing all the rubber parts in the pump) will
not prevent water from flooding the bowl via an open through-hull if the
pump in left in the "wet" mode. In the absense of a vented loop, a
failed wet/dry valve (VERY common in Jabsco toilets unfortunately) could
allow water to flood the bowl even if the lever is in the dry mode...but
rebuilding the toilet won't cure that because the necessary replacement
part isn't in the rebuild kit. So as long the toilet is bringing in
flush water and pushing bowl contents out, rebuilding ANY toilet
(rebuilding a Jabsco for any reason is just throwing good money after
bad) would be a waste of time, energy and money.
--
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://www.seaworthy.com/store/custo...0&cat=6&page=1
http://shop.sailboatowners.com/books...ku=90&cat=1304