Did you do a soap bubble test to see if it's leaking at the top or base of the valve?
Either way, it seems more likely that the leak is at the other end, namely at the piston O-ring which is a wearing part.
__________________
FB
Chain-L site
An ounce of diagnosis is worth a pound of cure.
Just because I'm tired of arguing, doesn't mean you're right.
“One accurate measurement is worth a thousand expert opinions” - Adm Grace Murray Hopper - USN
WARNING, I'm from New York. Thin skinned people should maintain safe distance.