Bostitch T31 air leak thru small head hole

Question

I had this rebuilt by a shop, then put a kit in it myself after it fired one brad and re-leaked. I only opened the head portion, not the handle section.The leak is through the small hole in the head and only occurs when the trigger is not pulled, plus the gun does not fire. The plastic trigger does not appear bent and the gun has only seen light home use.The new small O-ring seems to bind, unlike the old one and none of the old rings or seals seemed to be worn or leaking.