Gas it up and then dribble some spit on it or use fairy liquid if your too posh.
Then look for the bubbles. If it is coming up the middle of the fill nipple, empty the bottle, remove the nipple and inside will be a piston with an o-ring.
Either replace it or flip it over, refit with loctite or PTFE (loctite is best).
Let it set then gas it up again.