I have a craftsman nail gun model number 351-18314. When i shoot a nail the hammer that pushes the nail stays down and you have to push it back into the housing. Can you tell me how to fix it.

Try ez test to check for proper operation. Remove nails and air supply, point nose of gun up then down again. The driver should not move from the top of cylinder position. If driver is moving up then down replace the driver oring. If driver oring seems ok then connect air supply and bump fire the nailer without nails against soft wood and door open so that you can see the driver and feeder moving. Bump fire the gun and make sure that the driver is moving all the way back up to top of cylinder and the feeder is moving rapidly back and forward. If movement seems ok then hold the nose down on soft wood and depress and hold the trigger. This will fire the gun but driver will stay at the bottom position and not return until you release the trigger or lift the gun. When holding the gun with the driver down position, no air should leak out the top or bottom of the gun. If air leaks out bottom, replace lower bumper, if air leaks out the top replace the upper bumper. Good luck

There should be a spring to return the driver. The mechanism needs oiling with pneumatic tool oil before every use. If it's not been oiled, it will stick.

Try spraying with WD40, then try working it loose or using one of the nails from the clip try push it it from the outside and tap it with a hammer to dislodge the stuck one.

I don't have that same model but my gun started doing something similar a while back. I was able to disassemble the head of the gun and found that the hammer or piston that drives the nail had rounded itself off and wasn't able to make good contact with the nail head. Instead it pushed itself on top of the nail and got wedged. It's nearly impossible to see the stuck nail or dulled piston head without taking it apart. Hopefully this will help.

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

Hi, W/D here.
On the side of the slide that holds the nails will be a tab. If the gun is empty, it will be close to the base of the slide, near the head of the gum. Grasp the tab, and pull it all the way back until it locks in place. Load your nails. Now, near where the tab that you pulled back is locked in place, you will see a (usually) silver button. Pull back slightly on the first tab, then push the button, releasing the lock. Let the slide push the nails into place, and you're ready to go. Place 2 drops of oil daily in the nail gun where the air line connects, then hook up the air line. Fire away!
Here is a link to an exploded diagram of your nail gun. Double click the diagram to enlarge it. On the right side of the page is the link to a free download manual. Enjoy!
http://www.searspartsdirect.com/partsdirect/part-model/Craftsman-Parts/Nailer-Parts/Model-351183200/0247/0732000?searchedModel=351-18320&blt=
Best regards, --W/D--
Please feel free to rate this carefully hand-crafted solution, made especially for you. Thanks.

sometimes the ram breaks off in the center and it's pushed down but has to be pushed up to fire the second time. see if the ram will pull out it should not. be careful unload the gun first.
or the return spring has malfunctioned.

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