My older 3 1/2' hitachi nail gun will shoot one nail, then the driver sticks and will not retract. I can push it back in, but it is a little hard to do. I thought it was a bent driver, but when i replaced it the problem persisted. there is no air leakage from the gun when the driver is in the stuck out position.

Have you disconnected it from the air source and deep cleaned the nail gun? Sometimes even a tiny bit of dirt can block the rails the rod runs in. Once cleaned add a small amount of oil to the rod runners.

If the nail will not advance, easy test will determine problem. Open door and remove the nails. Lightly tap the nose of the gun against coiled rubber hose or soft wood with door open. If the driver falls to bottom, the piston oring needs to be replaced. The driver should stay at top of cylinder. Next pull back the feeder to rear position and check for tension. The feeder should spring back towards you when you push. Finally, the check paws on the door should return to extended positon when you push on them. Replace springs as needed. Connect air to gun and rapidly bump fire the gun against rubber. The feeder should move back and forward just as quickly as the driver moves. Good luck

it could be you are using the wrong nails, the gun should describe the correct angle and size.

Nail feeder on this gun is both air and spring controlled. Air pressure retracts the feeder and a strong spring pushes the feeder back to front. Remove nails and leave side door open. Dry fire gun very rapidly against thick rubber pad and watch the feeder retract and return. It should move very rapidly all the way back and front again. If it appears to move slower than piston is moving then follow next step. Pull the feeder back manually with tip of screwdriver. It should be very hard to overcome spring. If spring feels strong then suspect feedpiston oring or air supply to feedpiston chamber is restricted with debris from inside of the gun housing or air is leaking where body of gun mates with lower nose. There is a small white oring that seals between body and nose base. Sometimes this oring will be over-compressed thus restricting air flow into feed chamber. Remove two allen screws at front of base to split body from base and reveal this oring. Parts are available at www.toolpartsdirect.com. Good holidays

check for the proper nail collation and then check the feed pawl to make sure it is working properly, also check to make sure driver is fully retracting,

You have one or more worn o-rings in the head of the gun. They direct the air to the proper parts of the gun to first drive the nail then to return the driver to the top of the gun for the next nail. Depending on the manufacturer and your model number, some companies make kits to replace the o-rings (always replace all of them). For some companies you have to get a breakdown of the gun and order each o-ring seperately.

Are you using full round head nails?

The hissing indicates your O-rings are worn out and need to be replaced. This is not covered by warrenty and is common for all nail guns. Lots of kits are available on Ebay or your local Hitachi service center should stock them.

Hope this helps,

Cheers

There is a spring that is broken or the joint just needs some lubricant that can help take care of this issue

This happens to my gun too. You probably need to oil the gun once in a while. Here is how you fix it:
Disconnect the gun from the compressor first don't wanna get hurt or anything like that. There is a pin that shoots the nail out well that pin is jammed so what you have to do is push that pin to the original position. using a flat head screwdriver would help you to push that pin back into place. Let me know how that turns out please. I pretty sure that is the problem with your gun. Make sure you lubricate the Nailer once in a while.