[JRTJH]

The anti-siphon valve is a part of the black tank flush system. It us usually connected to the PEX tubing under the sink. Its purpose is to prevent backflow from the black tank into the water supply while flushing the black tank.

This is what it looks like to help you locate it. Many are under the vanity, some behind the shower wall. It's the part that "hummmmmmms" when you use the black tank flush.

It is "too fragile" and is the "weak link" in the black flush system. Some people have replaced it with a brass anti-siphon valve with good results. So far, I haven't had problems. Thinking pro-actively, I guess I should replace it before I have a flooded vanity, but keep forgetting to check for a suitable valve when I'm at Lowes. Oh, did I mention: I HATE PLUMBING !!! LOL