Bruce, the head valve and/or the anti siphon valve stop the flow of clear water into the head. Therefore both of them would have to fail. I would check your anti siphon valve if I were you. It may be plugged or not open far enough to let a little air in and stop the siphon effect. If you want to be sure, test it, take the sea water line off the head and if you only get a small amount of water it’s working correctly, if you get enough to fill the bowl and more it is not working correctly. The anti siphon valve isn’t easy to get to behind the vanity but it’s possible. Sometimes the little duck bill valve gets sticky and just needs some attention. I admit I do not shut the thru hulls when I leave the boat so I like the degree of redundancy. Len
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