[JohnRPollard]

Denise,

Based on your limited description, it DOES sound like you're describing the anti-siphon vented loop in the raw water cooling system.

The purpose is to prevent raw water -- after shut down -- from continuing to siphon from your intake seacock, through the engine heat exchanger, and ultimately into the water lift muffler. Normally this raw water would be ejected out the tailpipe by exhaust pressure during operation, but absent the exhaust pressure after shut down,the siphoning could fill the waterlift muffler and backflow through the engine exhaust system and into the engine. Hence the siphon break.

If the siphon break is not working, until you get it repaired, you should quickly close the sea-**** immediately after shutting down the engine. (In reality, the vanes of the raw water impeller will often check the flow of the raw water, but it's conceivable they might not so best to be cautious.) Also, I don't see any reason why you couldn't replace the anti-siphon valve with a similar valve from another vendor -- i.e. it doesn't have to be Vetus.