Hello XXXXXXXXXXXXX,
Welcome to Just Answer.
Sorry to hear about your problem.
It sounds like an o-ring. I recently had a similar problem with my Porter Cable roofing gun. I went on EBAY and found a complete Porter Cable o-ring overhaul kit for about $15.00. It comes with instructions, all needed o-rings and lubricant. And I have to say it was pretty easy to do. I would recommend going this route.
If you need further help or clarification on this answer, please do not hesitate to reply to this post.
Thanks for asking your question at Just Answer.
Best of Luck, Brian
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Architect
Licensed Architect- 12 years, L.E.E.D. AP, Former Remodeling Contractor- 10 years