Edd, BOTH the anti siphon valve and head check valve have to be malfunctioning as others have said to have the bowl fill with clear water. Black water would be a joker valve issue if you use your holding tank. We also have an issue with the V berth sink on a port tack filling with water so I installed a 1/4 turn drain valve because the thru hull valve is so difficult to access. It only happens 7.5-8.5 knots. All good now because she has been sitting on her steel cradle for a month!!! Len Mitchell Crazy Legs 89 C&C 37+ Midland On.
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