What is this part and what does it do?

could be a pressure regulator

Brass item is a vacuum break in the black tank flush system to prevent a siphon from occurring! The plastic part on the left is a check valve.

They gave you both!

Chris

HappyKayakers wrote:
I have a fitting like that for my black tank flush system.

Thats what it is. A vacuum breaker to the black water tank flusher.

I had a similar device in my camper start leaking. It was a one way valve and yours looks about the same. Mine was part of the sewer flush line and the one way valve was installed halfway in the line and placed under the bathroom sink. The device was higher than where you connect your hose and even more higher then the black tank. So I just removed the device and replaced it with a threaded elbow. It is above everything so it's not going to backwash anything back into my hose.

I have a fitting like that for my black tank flush system.

it looks like a cutoff, and someone choose this way to change directions on the pex.

I've never seen anything like it in any of my RV's. I could guess what it is but it would only be a guess so I wont.

rcknrbn wrote:
Noticed some damp folded towels in the small cabinet under the sink in the bedroom and assumed I had a leak from the p-trap or something sink related. Turns out this (pictured) device has an occasional drip. I assume it's a vacuum breaker or something similar. Is it repairable? If not, where do you purchase one? What purpose does it serve? Just something I haven't seen listed on some of the RV parts sites.

Thanks in advance.

Never seen one before. There's a lot of threaded fittings shown, have you tried tightening all of them?

Is the round thing a knob?