Roger Long wrote:
I found the leak.

Shot my theory in the tail.

Very small and didn't cause the problem on it's
own.

Or did it...? Two problems, maybe?

The pump is dry as a bone around the diaphragm...

A failing/failed diaphragm wouldn't cause the pump to leak...it creates
an air leak in the pump that causes the pressure to drop, which turns on
the pump. As the tear in the diaphragm gets bigger, the pressure drops
more often and lower, causing the pump to cycle more frequently and
longer...till finally the pump can no longer prime.

even after pumping
enough to fill the holding tank 3/4 full with fresh water to flush it.

???...holding tank or water tank?

The sink drain "T" into the head intake line works great although it
means a lot of head pumping.

Most likely because the toilet is pulling air through the sink that's
preventing from priming. Put a plug in the sink when flushing with sea
water...when flushing with water from the the sink, the sink needs to be
at least half full. Just running water down the sink drain won't work
'cuz the toilet will pull more air than water.

--
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