How to replace a cylinder ring in a hitachi nr83a

If its constant leaking inside the nailer (not between cap and body to outside feeling an airstream in front or on one side )then there is an problem between top seals VALVE and seals o-rings on top of the piston cilinder the parts on the top of the cilinder Seals and in the top in high priority must all be replaced because of the building up heat overtime the gaskets seals and o-rings are getting hot and hardened out over time there are only separate parts selled one by one to get I have not seen hitachi gasket/o-ring kits for nailers there where only complete kits for the older range of demolition hammers , but in the new models not anymore outside the States m,aybe but in Europe not. replacing the piston bumper could sometimes help extra because of the little height difference of the cillinder .

If leaking between cap and body with a palpable airstream and before the seal was closed tight , retapping the treaths some greater in the magnesium body and place helicoils inside it could be bad treath in one of them , it must put firmly fixed that will not fasten enough that was with the Brand Dutack Pro a high priority problem for a few years ago exact the same a few years now its modificated and that problem is gone.
Hitachi NR83A 3 1 4 34 Full Head Nailer Parts

maybe its your connection thats leaking and not the nailer.

Assuming you have an NR83A
The main reason the driver won't return is the Cylinder exhaust ring is missing, broken, or improperly installed.
Also check to make sure there is not a nail jammed in the nose inhibiting the driver from returning.
My recommendation is to install a Service Kit #18000 which includes the exhaust ring.

It appears that there is a small o ring that has missed you attention in reassembly and has fallen out of the groove. re do the job paying to gaskets and o rings.

Your problem is most likely where the cylinder / head come together. In the Hitachi framers, the cylinder is designed to move up and down (unlike Bostitch where the head valve moves) so if the O-rings are dirty or broken, or the spring under the cylinder is broken - the cylinder won't make a good seal to the head valve.
You can try plugging the tool in and giving a couple of firm taps straight down on the head with a rubber mallet - if the leak stops or slows you'll know the problem is with the cylinder movement.
Check the head valve seal - often they get sand / debris embedded in them which prevents the cylinder from making a good seal.
It is rare for a Hitachi trigger to be a problem.

I dont recall that there are any orings in the head area. Large leak from head area is caused by cylinder not retracting and seating against headvave. Small leak is usually gaskets between headvalve or cracked tube above headvale or rubber tube seal not seating against body of the gun. Test cylinder movement. Remove the head and push down cylinder, the cylinder should retract. If the cylinder is already down, pull it up. If the cylinder is very hard to pull up, most likely orings too tight or body of gun is warped. If the cylinder moves up with just little force, reinstall head and retest. If small leak, remove head and remove headvalve and look very closely for cracks in the head and tubes.

If orings fit too tight cylinder will not move against the headvalve seal (headvalve is the black rubber or vinyl seat in the head). To help the cylinder reset, bump the head against rubber matt. If the cylinder resets, good to go, if however the cylinder does not move into position, remove the head and check that cylinder moves up/down about 1/4 inch. If it does not move pull cylinder and use 400 grit sandpaper on cylinder where orings contact. Remove all scratches and reinstall. Good holidays

if you took the trigger apart the part that you unscrew has a nylon ball and a black rubber ring you should check them and make sure that you didn't loose them when you took it apart,if they are there, then you need to service the gun, and after you clean all the moving part and you put it back together, it may still leak from the top,Take a rubber hammer and give it a good hit,now not to hard, and it should seal

There is a small ball and a small rubber tube in the trigger as well, are the still there?

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.