Hi Just thought I would add my two penny worth. The seat the valve stem seals on is like a tap seat ie brass and gets pitted, resulting in dripping jets! I had a high pressure valve stem with a nylon seal, my wand is not high pressure so I changed it for a rubber seal valve stem which being softer stopped the leak. The 'o' ring controls the water going up the stem and leaking OUTSIDE the kingston valve.
Off course being brass over time you will need to change the unit when a new valve stem doesn't work.
Cheers Paul