[gss036]

I don't think there is an anti-siphon valve installed. There are 2 outlet/pickup tubes going into the tank. Both are within 1/4 inch or (probably) less of the bottom and nothing on the bottom of the tubes that I can see, they just look to be open. One of the tubes is plugged on top of the tank.
Where would the anti-siphon valve be located?
When I bought the boat new in 1989, there was a hose connected to the second connection and they just left it laying on the bottom of the hull with an open end. I removed it and put is a plug into the tank outlet.