water from my washing machine backed up and exited through my toilet. When the plumber came out, he discovered that the flapper of my backwater valve was out of its hitched groove and partially blocking the pipe leading away from the house. He cleaned it off, placed it back in its groove and called the problem solved. When I asked how I could be assured that it wouldn't happen again, he said that the cap to the valve keeps the flapper in place, so it shouldn't come off. However, when the plumber first opened the cap, it WAS screwed on tight, but the flapper was OUT of its groove. How did this happen? How can I prevent this from happening again? Is there a more reliable valve I can install?
This is a NDS system that is buried in my driveway about 2ft. below the surface.