In my experience, with similar issue, was not an actual valve leak, but rather a certain amount of fluid remaining in my long galley drain pipe. We all know that when dumping, the coach is not always sitting level, or leaning towards the dump hole. In my case it may be about a quart of fluid, next time I remove the sewer cap.
I tested mine by opening cap, drain in bucket after returning home. Yup about 16-32 ounces would come out. Now I would add water back to all three tanks, leave it for a couple days, open sewer cap...nothing, no leaks. So next time out camping, pull up to dump station, open sewer cap...nothing. When returning home, after dump, open sewer cap...yup 16-32 ounces run out.
I did buy a twist on valve, in case I really need it, or want to use 2 40 gal grey tanks as one. My solution was to buy a new sewer cap, with garden hose attachment/with cap. Now when ready to dump, I hold hose under garden hose cap, loosen slowly, let the 20 ounces go into sewer hose, before opening 3" sewer cap. Now hook up sewer hose, and start dump process.
Just a simple way to avoid a mess, and keep shoes dry!
Jerry