What is this?

[wilberb]

This is a picture from under my bathroom sink, the little piece at the top leaks water when traveling down the road. What is it, and what is it for.?

[bubbles]

If you have a black tank cleanout that is the backflow preventer and it should be rotated 90 degrees to the right (if that view is straight on). That puts the vacuum breaker at the top.

[fonzie]

Originally Posted by bubbles

If you have a black tank cleanout that is the backflow preventer and it should be rotated 90 degrees to the right (if that view is straight on). That puts the vacuum breaker at the top.

X2, here is general plumbing diagram....

[Bama Rambler]

There have been several reports of those things leaking. The good news is, they're pretty easy to replace.