I have had leaks several times from the same area. Each time I found the knurled nut needed to be tightened. Not sure if the seal was wearing or the nut just loosened over time. I remembered a tip someone once posted about using 2 hose clamps with a bridge between them that prevents the nut from loosening and I tried that the last time I tightened the nut. Very minimal leaking so far but I haven't put on too many miles since doing that. One hose clamp goes around the knurled nut, the other just forward of that on the ball tube. I used a slightly stepped piece of metal clamped between them thus preventing the nut from turning.
