Hitachi NV45AB coil nail gun is leaking air from top of gun

Remove the 3mm roll pin holding the trigger in place and set aside the trigger plunger (white plastic plunger should have an oring on groove). Remove the fitting that holds the trigger plunger and replace vinyl ball #875645 or check for debris under ball. Good luck with your repair.

This gun should be bump-fired only. Do not place the gun on the work and press trigger. Make sure that the safety spring is in place and has good tension. The spring helps to push the foot off the trigger so that it fires once. You could also adjust the screw that pushes on the firing valve. Turn in or out several turns till head of nail is correct when bump firing then lock the nut. The safety foot fits rather loose on the gun and should still work ok but can get so loose that it falls off the nose. Good luck

The two hex head screws on the very top of the head hold the air deflector and attach to the headvalve (the part inside the head with rubber coating, this headvalve has a gasket under that should be check for proper alignment ). Other small screws that look like set screw are only plugs that are removed to install auto control hoses in factory work. These are only plugs so just put them in a few turns with locktite.

The allen screw/s that you mention are there to plug holes where automatic equipment would normally connect (such as automatic nailing machines) . This is only a plug and should not be driven in too far. I have also seen allen screws installed in the head to repair small crack/s.

This problem could be caused by the head gasket fliped the wrong way not sealing small hole in the body. Also the head valve gasket (small round gasket between headvalve and head) could be place with vent hole mis-aligned. Check these two items and post again.

Hitachi nailers use an unusual design of actually moving the cylinder to fire instead of moving the headvalve. If the cylinder gets stuck in the open position (down), air will leak out of the head. To reset the cylinder, remove the nails and connect air supply. With air connected, lightly rap the head of the nailer against coiled air hose or soft rubber (too hard of a hit could damage the head). If after a couple of hits air still leaks, remove head and check headvalve for cracks. Replace as needed. If headvalve rubber looks ok, remove cylinder and clean/lube. If cylinder has fine scratches where orings make contact, sand smooth with 400 grit sandpaper. Lube with silicone paste, white grease or nailer oil.

Remove the nails and air supply. With side door open notice position of driver. Driver should be at top position with tip not visible. If driver is down and it moves within cylinder when you tilt nose up then down again, time to replace orings. If driver is down, it should extend past the nose about 1/4 inch (if driver is broken at the tip the end will look rough and nail will not drive in all the way). If driver extends past the nose more than 1/4 inch then lower bumper may be breaking up /cracked. Easy test for proper function. With nails removed and side door open, connect air to gun and dry fire gun against hard rubber or old coiled air-hose (with air connected and trigger depressed, no air should leak from trigger area, if air leaks when trigger is depressed, look for fault at trigger pin). Feeder should move back then forward in quick motion almost matching speed of driver and driver should not creep down between shots. No air should leak from nose of the gun (push nose against hard rubber and depress trigger and hold in this position, driver should stay down with very little or no air leaking out of nose, when you release trigger, driver will return). If air leaks out of nose while holding in this position, replace lower bumper. If a quantity of air leaks from valve in front of trigger, cylinder orings may need replacing or outer part of cylinder has fine scratches (remove scratches with 400 grit paper). Parts are available at www.toolpartsdirect.com. Good lucki

Hitachi nv45 series is the most popular line of nailers amoung professional roofers. There are three areas that develop leakes. Leak out head vent indicates cylinder stuck in lower position or headvalve worn. With air connected, gently rap head of gun against rubber mat to move cylinder to position. (Use caution not to damage head). If this action works, gun needs cleaning internally or lack of oil. If air still leaks out vent remove head and examine valve in head and replace as needed. Check that cylinder moves freely up/down (travel aprox 1/4"). If cylinder is hard to move pull cylinder and clean replacing orings as needed. If leaking around trigger, remove rollpin and trigger and check nylon ball for pits and replace as needed. If leaking around base of gun check for loose bolts and/or worn gaskets. Remove nail canaster to check feed piston gasket and screws. Good luck with your repair and post again if you have other question.

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

This probably is the problem,

The cylinder that the piston moves up and down inside is probably to low, and not making contact with the rubber piece in the head cap.

The cylinder has a spring that allows for movement up and down, take a pliers and gently pull up the cylinder if it does not move freely on its own. It will stop when it reaches the cylinder plate ( the part with 2 screw holes for easy removal). Be careful not to mar the cylinder though.

When reassembling the cylinder should make contact with the head cap. Hope that works!