Hitachi nr83a leaks air at new g.asket between head cap and body. screws are tight

This is not a common problem for your nailer. Indicates a small leak in air path from triggering valve to head cap assembly. Assuming that there are no external air leaks, I would clean and lube the cylinder. Try to smooth all the fine scratches that you see on the cylinder where orings make contact. I use very fine sandpaper and turn the cylinder with a drill attachment. (you can be creative with an attachment, like a round piece of wood 2-3 inches in length that fits snugly into the cylinder find center and screw in a 1/4 in diameter screw, cut off head of screw.) Place your homemade attachment onto drill and slip into cylinder, the wood attachment will not damage the cylinder. I use a variable speed drill and turn slowly while I hold a strip of sandpaper against scratched area. Just a light even sanding is all that is needed. If not too scratched just leave as is. Replace oring number 877316 for cylinder plate (outer) and 877327 packing and lube with white grease. You do not have to replace any other orings unless they look damaged. If you replace all the orings, the cylinder will be too tight and my not work at all. Make sure that the three bolts on top of head are tight (no need to remove them, just make sure they are tight). Good luck.

If you are going to replace the cylinder ring I would recommend getting the service kit (Part #18000) and replacing all the o-rings.
I'm assuming you have already taken it apart this far but if not:
Remove the 4 hex bolts on the cap and remove the head.
Using 2 of the bolts - thread them into the inserts on the top cylinder ring and use them to twist it off.
The cylinder and piston should 'pop' up under spring pressure from below.
Using an o-ring pick, remove the o-rings and thoroughly clean the tool - replace the o-rings and the bad cylinder ring. Lubricate the new rings with the grease provided and reassemble.

Hitachi NR83A 3 1 4 34 Full Head Nailer Parts

maybe its your connection thats leaking and not the nailer.

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.

NR83 is the stick nailer. There was two versions. The NR83A was for full round head nails and the NR83AA was for the clipped head nails. The coil nailer was the NV83A was the coil nailer. Here are links for the parts diagrams of all 3 so you can see if one of these is what you have.

http://www.toolpartsdirect.com/cgi-bin/schematic.cgi/hitachi/NR83A

http://www.toolpartsdirect.com/cgi-bin/schematic.cgi/hitachi/NR83AA

http://www.toolpartsdirect.com/cgi-bin/schematic.cgi/hitachi/NV83A

Often a trigger leak in the NR83A is caused by a bad gasket in the head #877-327. If this is improperly installed, cracked, or overcompressed it will cause the trigger to leak.
It is uncommon for the trigger itself to be the problem.

My Guess is that it needs a new "O " ring or some other form of seal some where round the trigger actuating spindle. Should be quite easy to fit if you can get one the right size

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.