I own a hitachi nr90ac3 framing nailer and i

Should take up to a 3 1/4 in. nail

Just as a matter if interest, were you using the tool in soft or hardwood? This is something that only happens if repeatedly used in an extremely hardwood like Ekke or similar. Did Hitachi resolve the problem with you?

Regards
Alex

The piston is broken if it came out.
Remove the 4 bolts on the head, remove head and you will see the piston - be careful removing it because it is easy to score the cylinder walls if it is cockeyed.
Replace the driver, lubricate the O-ring before inserting

use the lube that came with the kit, you can replace everything correctly but if oring gets pinched it will still leak

This is most likely caused by the band (or o-rings) on the outside of the cylinder. It could be a trigger valve problem, but the fact that it doesn't come out until the trigger is pressed points to the cylinder rings.

New user will jam the gun/ nails because gun was most likely not bump fired. Placing the gun then depressing trigger will usually cause double fire in the hands of new user. New user should practice on scrap lumber bump firing at many angles till they get the idea; or give them a training period with a nailer that has a single shot trigger (they may develop bad habits with single shot however).

Over time the orings within the nailer will dry out causing problems. Adding oil thru the air inlet will not get oil to all the orings. If the gun had been in storage, add a few drops of oil into air inlet and spray silicone (or teflon) spray lube into air discharge opening at the top of the gun (it is difficult for oil to get into the head from air inlet). Before you connect the air, remove the nails and make sure that the drive piston is at top of cylinder. You can verify by inserting long metal tool (slightly larger in diameter than nail shaft) into nose of the gun and tapping firmly. This will move the piston up against the headvalve and hopefully reset it and allow oil the lube orings. If the headvalve is stuck, gun will not fire. Make sure that the safety foot is depressing the firing pin at trigger valve all the way in. Retest gun for proper action. If still no joy, remove the head and lube orings with silicone paste lube(silicone past has better sealing action but dries out over time). Good luck

Have you used any kind of lubricant on the nailer?

sounds like the wrong part clickhere these cats got a good list of parts
if' i was of help tell me so i know i didn't lead you astray..
thanks jy

Several models of hitachi nailers share the same orings and other parts including the check ball under the trigger pin in your gun. Remove the roll pin holding the trigger to release the trigger pin, (plastic with one oring) then remove the valve with a screwdriver. Replace the vinyl check ball and seal. Parts are available at www.toolpartsdirect.com. This should fix the problem that you are describing, however use caution and check that the safety if properly adjusted. Enjoy