The face seals are different on RTP's with the drop out. Those are the 007 instapierce pins, not the traditional Sheridan valve style.
Like Funsi said, if it's the valve o-ring you'll need a valve tool. (if you're going to be keeping this marker, or have a hankering for any other brass, you mid-as-well pick on up from Titus anyway).
Where's it leaking from? Down the barrel? Around where the 12 gram is seating? That would help a little in diagnosing it.