Put a drop or two of oil up top by the valve pin and screw the tank back onto the marker then take it off. This should fix the small leak.
Image of what the valve body looks like taken apart (Do NOT do this, just showing you what is happening). Not taging it to keep the thread looking decent.
Look at the enlarged picture on the bottom and take note to that Oring on the far left, this oring is not sealing for whatever reason and odds are it just needs to get lubricated which is what you're doing with the above directions.