Lay the gun down (prop it up) so that it is sitting on the front of the
gun with the end of the magazine and the air coupling pointing straight
up. If the gun is dry and that is the leak source, put 10 - 12 drops
of oil in the air coupling and let it sit propped up for an hour or so.
The oil has to work its way to the front of the gun, then it will get
drawn into the piston and onto the "O" rings.
You didn't say how old it was, or how much it has been used, or if it
received any routine maintenance. You may have blown a cylinder ring
or even have a leaky head gasket.
If it is just a dry ring, you are lucky. If it is the oil required
gun, then you should put a couple of drops in the gun at the start of
each session, and if you are using it a lot, put that much in again
after using it for a few hours. When trimming, I put oil in first
thing in the morning, and again after lunch.