Anti-siphon valve?

Hooked up the black tank flush for the first time this season and pumped a few gallons of water into the sink cabinet. The line (from the outside connection port) runs up the bottom of the sink cabinet into a black plastic apparatus and then down into the underbelly from there. Is this thing possibly an anti-siphon valve? Are they prone to leaking? The water seems to be coming from that location, but I am unsure. Thanks for any help. Mark

It is an anti-siphon valve. It is supposed to open when there is no water pressure so that water on the black tank side will drain into the black tank. If it does not close when you apply water, it will leak. Some have failed, broken and leaked.

And I think many who can find them remove them. I finally found mine on our 3010 up behind the shower. Before our next trip it is coming out.

I understand the reason for it but I always have a valve on the inlet to the flush system so there is never any back flow.