Following advice of a homebrewer at Praxair, I tightened a nut in the middle of the valve handle with a manual nut driver (if that is the right term, it looks like a screw driver) so I would avoid overtorquing it. That sealed the leak. I think the nut tightens the packing around the valve stem. I think the leaky stem may have allowed some air in the gas line which caused my kegged beers to age really fast.