Black Tank Flush leak not check valve, wrong!

[Carl n Susan]

The standard location for the black tank flush anti-siphon valve in a 3400 is behind the shower. There should be an access plate from the closet area. Remove it and the valve should be viewable. The access hole is often not directly by the valve so you may have to reach around or enlarge the hole.

Lots of folks have had the problem and there are lots of discussions (with pictures) to be found using the Search function.