Lets assume you have oiled the gun. Before you run out and buy an o-ring kit for this, take the gun apart and look for anything that may be jammed inside the gun. This could be dirt, grit or an unfired nail. The gun is very simple inside so don't be afraid to do this yourself. Undo the 4 hex screws holding on the cap at the back of the gun. Put the nose of the gun in a vise or get a friend to hold the gun for you. Grab hold of the cylinder inside the gun and pull it out. Wiggle it around and pull at the same time, it will be tight but it will come out. Totally disassemble the cylinder. If the seals look good take a very close look at the "Bulkhead". It is made of plastic and is prone to get very small hairline cracks in it and this will leak air. Look inside the cylinder and make sure the walls are smooth and not scored. This is all very easy to do. Don't just buy an o-ring kit assuming it will fix the problem. Look for the simple stuff first, it may save you some time and money. AGAIN IF THE O-RINGS LOOK GOOD TAKE A VERY CLOSE LOOK AT THE BULKHEAD AND LOOK FOR CRACKS, IF AIR IS LEAKING FROM THE NOSE THIS SHOULD BE THE FIRST PLACE TO LOOK AND COSTS ABOUT $7.00 TO REPLACE..........
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Dewalt nailers have some vinyl orings that fail. Usually found within the trigger valve, headvalve, and between the cylinder and cylinder spacer (white plastic piece). A dewalt nailer with failing orings may not leak air however. The vinyl orings tend to start to crumble and the tacky-crumbling pieces tend to create a seal. Easy way to check condition of the vinyl seals in a Dewalt is to remove the trigger valve and look at the orings on the firing pin/stem (2mmx1.5mm orings). If these two orings are soft and sticky, then orings inside of gun most likely the same. Check band on the outside of the cylinder (part # 390159-00) is what causes piston to return to top. But also check to too tight or too loose piston oring, bent shaft of driver and worn/ broken lower bumper. If all is well with internal parts and piston does not return to top, too much air is leaking past nose of gun. Oring kit sold by dewalt will contain most of the orings needed. The kit may not have orings for the trigger. You will at need two orings (2mm x 1.5mm) for the trigger valve stem or just replace the valve. Use white grease or silicone paste lube on all the orings. Good luck.
Dewalt framing nailers use some vinyl orings (gold or clear in color) that fail too soon in both the trigger and the cylinder spacer. Air out the nose of gun indicates air is leaking past the cylinder spacer seals (If piston is pushed down with air leak then headvalve seal is leaking (remove nails for this test)). If piston is not pushed down with leak, remove head and pull out the cylinder with cylinder spacer (white plastic). Place the cylinder on workbench with bottom up. Push down on locking ring and remove retainer to release all the other parts. Take picture or take note of parts together and as you pull them off to aid in re-assembly. You will need to replace the two orings between the white cylinder spacer and the cylinder. Order oring kit from dewalt or just purchase one ( # 141 universal oring ) and one metric oring 74.4mm x 3.1mm from hydraulic parts store. If the piston was pushed down with air leak, replace headvalve orings in the head. Good luck with your repair
Dewalt guns use between 70 and 120 psi. I use 100 psi on all framing guns when testing, however the nailer will last longer if you use lower pressure (only whats needed to drive nail into work, depending on type of wood) . When too much pressure is used, extra force is absorbed by the lower bumper and nose area of gun. The address below is dewalt service and has all the info about your nailer. Just enter the model number and look for pdf file. Check your model number for safety recall. Good luck www.servicenet.dewalt.com
Either the plunger has not completely returned to the "pre-firing" position, or there is a defect in one of the o-ring seals. Make sure that the gun is sufficiently lubricated and if so, then next check the condition of the o-rings. An improper seal will cause this. It is also possible that the trigger assembly has a small crack/leak.
Don't know if this will help you but here goes. I have a Bostich nailer and had to replace the check valve under the trigger.(at least it acts like a check valve, stops air loss when disconnecting gun from hose) It was leaking badly and had to remove trigger to replace the kit. You can do it yourself like I did or take it in to a shop. I carefully punched out the roll pins, replaced the kit after greasing the o-rings with lubricant that was supplied with kit and replaced the trigger. Good luck
Generally, when you attempt to fire and only get an air sound at the trigger, the problem is air leaking past the orings in the head valve. Here are a few things to check and a simple test.
Remove the trigger from the gun so that you can have access to the trigger pin. Remove the nails from the gun. After removing the trigger and the nails, reconnect the air supply (check for 100psi) Place the nose of the gun against wood and with a very quick ****, depress the trigger pin all the way in. If the gun fires then the safety foot is not allowing the trigger to depress the pin all the way in. Make adjustments to safety and check for worn trigger and trigger retainer pin. If the gun does not fire, remove the head. Place the head on table and depress the headvalve all the way in with your thumbs then place a finger over the small vent-hole on the gasket surface and release thumb pressure. The headvalve should remain depressed due to vacuum in the valve and will pop up when you remove your finger on vent hole. If the headvalve does not stay in, air is leaking past the orings. Remove headvalve parts and use magnify lens to find small scratches on the surface where orings contact. Remove scratches with very fine sandpaper (300 - 400 grit) Make sure all the orings are not cut (use magnify lens) and fit snuggly. Lube with "Danco silicone grease" available at lowes in the plumbing department. It is used to lubricate orings in faucets. After re-assembly, do vacuum test as described above. If you have vacuum, head is ok. Make sure that the gasket seals around the vent hole where the head mates against the body. You should be good to go. Good Luck and email if you have other questions.