Was it locked up where nobody could get to it?
Next, some valves leak if the cap is screwed on too tight.
I had to squirt soap on mine and tighten it to that special place where it wouldn't leak till I got a good one.
I used a cheap Chinese valve.
Some caps have a rubber seal at the bottom. But all it does is push the pin and make it leak.
Make sure there is a hole for the pin to go in at the bottom of the cap.