I just installed a new cap on the Korky fill valve in a Kohler Wellworth toilet, along with a new flapper. I left the lid off for a couple days to watch the water level and was surprised to see that it rises very slowly. When the valve shuts off, the water level is just below the water level line on the tube. When I check it hours later, the water level has risen. Overnight it rises to the top of the tube and undoubtedly overflows into the tube, although I haven't observed overflow. I don't hear any running or hissing or see any bubbling. Do I need a new fill valve?