Sea water filling toilet bowl...

[KFuller]

We have a new-to-us Belize 43 catamaran that we are living on. She has two electric Jabsco heads that work great except the port head will eventually fill with sea water if the thru hull valve is left open. Closing the supply off stops the flow. The water will encroach into the bowl until just below the rim and stop (probably about sea level). Previous owner says this was not a problem in the 5 years he had the boat. I have considered extending the supply line to add an above sea level loop but wanted to see if anyone can tell me what the real cause could be. I am assuming something in the pump is allowing the bypass but cannot figure out what or how.

Thoughts? Suggestions?