Users of hand pumps (specifically G6 Venturi) Help!

your not missing a piece but the oring might not be sealing properly
​replace the oaring and make sure the screws are tights id also suggest little silicone grease over the orign to make sure it sealing
​the pump draws air in threw the handle so that should be a 1 way valve if you really think its leaking threw the top then its most likely a bad seal on the valuing inside the tube not the handle

I think it's fair to say that hand pump owners would benefit from doing the service on your own -- as preventative maintenance. From what I hear the Venturi is one of the easier ones to rebuild, you're fortunate.

I have a Benjamin pump, not by choice, and only after a couple of months it failed on me. I called the company who does the repairs and they were very good in dealing with the problem. They offered to take it in for service but also gave me the option of doing it on my own.