My experience has been the valve shaft wore and became a bit loose in the hole. I just assumed it was from the carbon buildup that created the friction that wore the stem. So I used to clean them a couple times per season just to be sure. I would have assumed there would have been a wear limit spec, but the shop manual says nothing.
If you do buy a used assemble make sure you get the newer version of the pistons and they match what is on the other side. I had two different versions on mine when I bought it.
But as etemplet said, I would not worry about it either. Mine also leaked liked yours and it did not effect performance at all.
Cheers