Just replaced o rings and piston bumper, but gun still leaks air (out of nozzle) and wont fire at all

Air leaking from nose of gun indicates air getting past bulkhead orings. Bulkhead is white plastic frame inside of gun. Replace gold colored vinyl orings with like size buna orings. Remove cylinder from bulkhead to find two bad orings. Also replace trigger valve or vinyl orings within valve including the two orings on the firing pin. Good luck.

Check band # 175520 (black rubber band around the outside of the cylinder) is what causes the piston to retract. If the check band is in place, then problem could be caused by cracked lower bumper, worn piston oring or cylinder seal on the headvalve. IF the cylinder seal is bad, air will leak between seal and the top of the cylinder and could push piston down cylinder. If piston oring is too loose, the piston will not stay at the top of the cylinder. If you point the nose of the gun up then down again and the driver moves within the cylinder, replace the oring. Good luck

Glad that you repaired your gun. The piston oring is the only oring in the gun that can have slit and still work fine. It works like a piston ring in a motor. Small ring gap is ok just as long as the piston does not drop down the cylinder under its own weight. The piston oring does not hold pressure, it only reacts to pressure when headvalve opens. When this series of gun leaks air, it is usually due to headvalve orings (lube with white grease), Cylinder seal loose or broken, or trigger valve oring splits (tva6 valve). Good luck

Make sure that orings are in good shape, lower bumper not cracked, (no leaks and the driver/piston stays at the top position, if piston drops a little or all the way down with no nails in gun, replace piston oring). Also check spring tension for feeder and two check paws in the door (feeder tension should be firm ). Adjust the timing screw up or down one or two turns and test (timing screw at the top of safety foot, head of screw pushes on the firing valve ). Test for lower bumper cracked. Remove nails and leave side door open. Hold the nose of the gun against coiled air hose or thick rubber and press and hold the trigger. The piston will stay down against the bumper until you release the trigger. No air should leak out the driver path. If air leaks past driver suspect bad bumper. Test action of feeder piston. Remove side cover to feed piston and clean debris. With air disconnected, move the feed piston back and forward. There should be a little resistance, if it moves very easy, suspect orings worn. As soon as you connect the air line. the feed piston should move forward with force, note the position of the feeder. Now with air disconnected, place nails on the feeder. The first nail should be well within the path of the driver. If not fully under driver path suspect worn feeder / feeder retainer pin, or debris inside feed piston chamber preventing piston from full forward travel (debris from inside of the gun will enter the feed piston chamber usually broken bits of lower bumper). Replace parts as needed. Good luck with your repair.

Remove nails from the gun and dry fire the gun by bumping it against thick rubber mat or coiled up rubber hose. As the gun cycles notice the feeder moving back then forward in time with the cycle. Make sure that the feeder is connected to feeder driver with a roll pin and that the check feeder is in place in the latch door. The check feeder is thin metal and should have spring action. The driver should return completely to top of cylinder with some degree of force. There should not be much air leaking out the nose as the gun cycles. If a quantity of air is leaking out nose with each cycle, then lower bumper may have broken/cracked. Replace as needed. If very little air is leaking out nose with each cycle then driver could be bent, piston oring could be worn or lack of lubrication in cylinder. If all lower parts seem good, then only area left to check is head valve. There is a 1/4 inch dia. rubber ring at the very top of head that could restrict air flow out of exhaust port. Adjust as needed. Good luck.

For nailer to retract properly the lower bumper should not be missing any pieces or cracked. The upper bumper should be pliable and complete. Pull the cylinder and check for a groove cut into cylinder with a series of holes. This groove should have a snug fitting oring. It should not take much force to move the piston within cylinder. If all seems good, re-assemble using silicone paste lube available at Lowes or swimming pool supply. Easy test. With air connected and nails removed, dry fire gun against coiled rubber hose. Keep holding the trigger and keep the gun against hose. The piston should stay down and no air should leak from gun while holding in this position. Lift up gun and the piston should retract. If air leaks from bottom while holding in this manner, lower bumper is not sealing, if air leaks from top, then upper bumper is allowing air to leak from head valve. Good luck

Miss fire usually occurs when piston/driver is bent, piston oring worn, lower bumper cracked or missing pieces or upper bumper is cracked. Easy to check. Remove head and pull piston. If bent try to adjust, but be careful, blade breaks easily. Piston should fit snugly in cylinder, if loose, replace piston oring. If oring/plastic ring ok examine lower bumper for cracks and replace as needed. If lower bumper ok, pull out headvalve and examine upper bumper for cracks. If upper bumper ok, pull cylinder and look for loose check band around cylinder. Check band is black rubber and should be placed around cylinder fitting snuggly covering series of holes. Replace parts as needed and lube with silicone paste sold at Lowes in the plumbing department (or white grease). Good luck

Causes of piston not retracting are as follows. Cracked /worn bumper, if cracked air will leak between piston and bumper. Try test, remove nails and dry fire gun against rubber or soft wood; depress trigger push gun against wood/rubber and hold in this position. Piston should stay down and no air should leak past nose. Worn piston oring, piston should not slide down cylinder under its own weight. Loose or missing check seal, remove cylinder and look for black rubber band around series of holes. Band should be in place and fit snuggly. Good luck

You either have a piston stop bumper failure or some damaged orings. Look on the schematic and the piston stop bumper is number 11, part #107236. A lot of times the piston stop bumper will disintergrate due to becoming brittle from the compressed air and not having enough lubrication. Look for small plastic balls in the head under the valve. If you see little plastic pieces than that is what remains of the piston stop bumper. You will need to remove the valve from the head cap in order to inspect the bumper. Also inspect all orings. The slightest damage can have big effects. One other thing to look for is damage to the plastic valve that rides on 3 orings. (2 out and 1 in) I don't believe you have valve damage because you say it is firing like a machine gun. This is almost always the piston stop bumper.

The Hitachi nv45 series guns are lifetime guns and parts (parts available at www.toolpartsdirect.com) are not too expensive. There should be no air leaks for best results. If air leaks from the trigger area remove the trigger and white plastic plunger. Make sure that the plunger has oring and the ball that the plunger acts against is whole. If air is leaking from the boot around the firing pin then remove the head of the gun and pull off the cylinder ring and check the outer oring. (After you remove the head, the cylinder ring is the part that has two threaded pull points, the outer oring sometimes breaks causing leak) With the head off push the piston all the way down and make sure that the driver extends about 3/16 past barrell. If the end of the driver is wrorn to round shape, you can grind it square if there is enouth length left. If the sheet seal is good the driver will not pull out of the cylinder.(look at parts diag. to find sheet seal, it is below the bumper) If the driver easily pulls out of the cylinder then open the gun from the bottom and examine the bumper and sheet seal and replace as neccessary. If the driver will not pull out of the gun (thats a good thing) then pull up and to side enough to peek/look at the lower bumper to check for cracks or missing pieces. The driver oring should hold the driver in upper position. Check that the top edge of cylinder is smooth and seals against the vinyl part in the head. Use caution not to damage the top edge of cylinder. The oring kit for this gun will cost about $20, however you only have to replace the worn parts for good results. Let me know if your air leak is somewhere other than stated above or other questions. Good Luck