My boat has 2 Raritan Atlantes Freedom heads. When I wet flush the guest head, you can hear the pump running for a good 5 seconds before water starts to flow into the bowl. A full wet flush lasts 10 seconds, so I'm not getting much water in there for a good flush, so I must flush again. The second time there's plenty of water and the job is usually done. About 15 seconds after it's complete, you can hear the water in the feed pipe gargling back into the lake. If you try to flush after that, it's the same thing all over again. I imagine I have an air leak in the system somewhere, but I'm wondering if it's the impeller. My boat is 6 years old. How often do those need to be replaced?
16x66 Sailabration on Percy Priest Lake near Nashville.
There should be a check ball in the incoming line right before the impeller. It is probably stuck. You can put another check valve at the through hull like OHB suggested. There should also be a debris filter before the check valves.
If you have been running the impeller dry for 10 seconds everytime you flush, it is probably ready to be replaced.