The way that seal works is by compressing the donut seal near that collar. Tightening the knurled ring is what does this. It requires a large pair of channel locks to do it right. If it doesn't want to turn with relative ease, loosen it all of the way and slide it to the rear and clean the threads with brake cleaner and give it another go.
If it still leaks after that, the seal probably has a crack in it.
Carl
