Thursday, March 30, 2023


Ask Dave: What is this leaky valve under my bathroom sink?

Dave Solberg is a leading expert in the RV industry and author of the “RV Handbook” as well as the Managing Editor of the RV Repair Club.

Dear Dave,
I repaired a couple of leaks under the bathroom sink. This connector intermittently releases a small amount of water. Not sure what it is and whether it is functioning as designed. Is it some sort of pressure relief connector? Thanks in advance. —Perry

Part 1

Dear Perry,
Thanks for the photo. I can definitely see the water drips on the side wall. I have never seen anything like this valve on the fresh water side. However, I have run into a few “cheater” valves, as we call them, for venting holding tanks. Holding tanks need to be vented just like a sewer line system in a home to eliminate a vacuum for draining. In an RV, the vent comes up inside an interior wall and out the top of the unit with a cap on top of the pipe.

If a holding tank is situated underneath the frame directly below a kitchen countertop, obviously the pipe cannot go up through the counter to the roof. That’s why manufacturers use an anti-syphoning vent cap. It’s designed to allow air to be drawn in with the water flow, but restrict odors from escaping out. I’ll need to do some research and will follow up. Can you provide the make, model, and year of your rig?

Part 2

Perry wrote back that it was a Keystone Hideout 24LHSWE travel trailer. So I contacted my Keystone rep and he identified it as an anti-siphon valve for the black water tank flush valve, Part # 397250. Most of the black water flush valves I have been associated with are a simple mount from the side of the tank sprayer with a water hose connector on the outside of the tank. This typically has a back flow prevention connection associated with it to make sure waste water from the black water tank doesn’t get overloaded and backflow into the fresh water system.

Perry’s system seems to have the connection valve in the service center and piping going to the black water tank. The valve at the system is just a simple garden hose connection, so they need a backflow prevention valve somewhere inline.

So it is not a vent, but rather an anti-siphon or anti-backflow prevention. It should not leak. You can order a replacement part directly from Lippert at

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Dick and Sandy near Buffalo, NY
1 year ago

Definitely a back-flow prevent valve so water meant for the flush out of the black water tank does not get back into the fresh water system. There are more costly brass versions also. Stay well, Stay safe.

Thomas D
1 year ago

Congratulations, Dave you got one right without going too far off subject. I was worried when you started about the sewer vent, complete with picture. But you were right on about the anti siphon. Good job man

Mark Moore Berlin, Maryland
1 year ago

I had a similar issue with the back flow valve on our 2021 Grand Design 2600 RB. While unloading things from our last camping trip I noticed that the bottom shelf of our narrow shelving area beside the shower was really damp. I thought that maybe we had spilled some liquid on it. It never got dry, so we just did not use the shelf on the next outing. I checked the shelf again after the next trip it felt even wetter than the pervious trip. I shelf was now starting to delaminate, and the face frame was also showing signs of distress. It was now “demo-time”. I removed the shelf and found the faulty back flow valve mounted on the common wall to the shower, about 2” lower than the shelf bottom. It had been spraying up on the bottom surface of the shelf, I guess every time I rinsed out the black tank. I removed the valve, added some fittings to each side of the “pex” tubing and simply bypassing the cheap plastic valve. I will not replace it!

1 year ago

Did the same 2, 90 degree fittings with a small piece of PEX and done. The air gap’s they put in RV’s are junk.

1 year ago

I have the same setup under the sink in my half bath of my Coachman Sportscoach.

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