Phantom Leak

[phillyg]

What's the whitish panel above the relief valve? It might be accessible from above. Or maybe cut the two verticle pipes lower down, afix the new valve with 90s and longer pipes and reattach.

[jdarwin]

The panel you see is (I believe) the underside of the floor. It’s plywood.

[Carl n Susan]

You will have to figure out what is above the calve. There may be an access port under the couch or in a cabinet. Montana tries to install the anti-siphon valve above the tank height. As long as the replacement achieves this you can do what you suggest.

[RMcNeal]

JDarwin, I'm glad you were able to find the leak. I only have 1 black tank, but I just simply removed the vacuum breaker/check valve and plumbed it straight through. Then I put a backflow preventer on the hose connection in the convenience center. My valve was under the bathroom sink and was leaking the second time I used it.
If you do replace the valve, I would at least get a good quality brass one instead of the cheap plastic one Keystone put in. I think you can find them at Lowe's or Home Depot.

[Mikendebbie]

Here are the locations for both vacuum breakers in a 3921FB...
The access hole is under the bed in the slide-out.
It is probably easier to cut the pipes, remove the breaker, plumb the pipes together - than to remove the bottom of the bed to reveal the access hole.

[jdarwin]

I found this leaking at the rear connection. Looking for suggestions to replace with brass.

BTW, I did as suggested and cut the lines, tossed the bad valve, mended the line and installed inline shark bite check valves directly behind the convenience panel. I also checked the valve in the master bathroom and it was OK.

[jdarwin]

Finally finished. Shark Bite is proud of their products. I posted a couple of pics for others in the same situation. Thanks for everyone’s help.