I know this thread is a few months old but I had to comment on my experience with a broken dump valve.
For me the part that was broken was the square frame piece that is glued onto the outlet of the black tank. Two of the corners that the valve screws go into had broken loose, but were still there hanging onto the screws. That left enough slack in the system to cause a slow drip when the black tank was nearly full.
When replacing both valves, which had leaky internal seals, I was able to repair the broken frame so it could be left in place and reused. I just cleaned both edges of the breaks very well, roughed them up with some fine sandpaper, and then stuck everything back together with some solvent glue that was rated for ABS and PVC pipe. Just to be sure, I let that joint cure for over an hour before reassembling everything.
That was several years ago and no problems since.