Sometimes my black tank dump valve leaks, and sometimes it doesn't.
At a recent rally, I had the blue-thunder hooked up, and it seemed that it was filling up rather quickly...I suspected that we might be leaking, although when I emptied the blue tank, it didn't look/smell funny or anything...
so when I got home, I filled up the black tank w/ water to do a test. I threw some of that valve-lubricant stuff in there, while I was at it, and just let it sit for a while. I checked it every couple of days, and the panel indicators always said it was full. (I also looked at the outlet for signs of a drip...nothing) this continued for about a week. Then when I checked it, the panel indicator showed that it was down to 1/2. I looked in back underneath, and sure enough, there was a wet spot on the ground, and a "drip, drip, drip" coming from the outlet.
I understand that when you dump, its possible for crud to get caught in the seal, which could cause a leak. But why would the tank seal up fine, hold water for a week, THEN start leaking without having moved the valve?