The O-ring is not sealing properly and is probably due to lime (or other mineral) buildup on the mating surface.
The O-ring size is 11/16" OD x 1/2" ID x 3/32" CS or replace the K0858 Torque-Free Valve Stem End.
Access the O-ring To remove the lime, saturate a paper towel with white vinegar and place it in the valve stem body. The vinegar will dissolve the lime. Keep the towel saturated until the lime is dissolved. Do NOT scrape the lime away. Scratches in the surface will cause a non-repairable leak. You can brush it with a soft brush such as a tooth brush.