There is a small ball and a small rubber tube in the trigger as well, are the still there?
95% of the time your problem will be in the head of the gun.
You said you put an o-ring kit in it, so next check the head cap (877-307) for any nicks or imperfections, replace it if so. The other area in the head is the 2 gaskets under the head cap - if you try to check them, you will have to replace them - they will tear when you separate them.
The other possibilities - one, one of the o-rings on the cylinder guide got pinched when you reassembled, or your gun housing could be distorted causing one of the Cylinder guide o-rings to not seal properly.
These are the most common reasons for a Hitachi trigger leak.
SOURCE: Hitachi NR83A air leak
Most all Hitachi nailers that use the plunger style of firing/safety mech will leak air past plunger (part #877-339) when firing, however should not leak when connected to air or resting. If air is leaking past plunger when connnected to air check following:
Remove head to reveal cylinder plate (part#877-318). Remove plate and check outer oring for nicks or broken. If nicked or broken, replace and problem solved. If in good condition, remove cylinder guide (part #877-310) and check two outer orings for nick /cut. Use caution when removing/installing cylinder guide. It is easy to pinch orings when installing causing air leak at plunger. If orings are in good shape then problem is warped body of gun not allowing outer orings to seal. To check for warped body, use the cylinder plate as a guage by lowering it into the area where the cylinder guide rests. Good Luck with your repair.
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