I have a system that will hold water over night but pumps endlessly. After a little research I think duck bill valve replacement is what needs to happen, but I don't know where the hide..

the vacuum switch was the problem with my system.it screws into the end of the vacuum tank and is an easy part to replace.

That is apparently the 3rd most likely issue. Maybe I should look into that next. So you pump the system out clean, and unscrew it from the holding tank?

it is not in the holding tank,it is in the vacuum canister.

any leak causes this, even the flat rubber under the bowl itself, the duckbills are located in the valve assy where the hoses are between pump and head, can be mounted anywhere. does the head keep water in it over night ?

yes that ball seems to seal. If you let it pump for a minute or so it pulls the water pretty good when flushed. The hose exits the head and goes straight into the boat until I see it in the engine compartment, not sure I can get to it or see it for about 10 feet.