Flooring nailer tech support

You just need an O-ring kit. It is letting air past the trigger and valve.

https://www.google.com/search?q=mfbn125+brad+nailer+o-ring+kit

Parts Schematic

https://www.google.com/search?q=mfbn125+brad+nailer+parts+diagram

Leak Troubleshooting

https://www.google.com/search?q=mfbn125+brad+nailer+air+leaks+past+trigger

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The trigger valve should release air when depressed. This release of air comes from behind the head valve. If the head valve does not pup-up when the trigger is depressed then air is leaking past the head valve orings. Try coating the head valve orings with plenty of white grease and retest. If the gun fires you know that orings are leaking. Test with no more than 100 psi. Good luck with your repair.

the top vent is the exhaust and usally leak from exhaust is trigger valve not seated properly if new ( oring s like to roll) or if not new sometimes internal spring breaks not having constant pressure on valve allowing to leak through exhaust.

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

Easy fix if air is leaking out the top of gun. Remove the head and lube with white grease or silicone paste. If orings seem to be loose, order repair kit from dewalt (you can tighten orings by removing the oring and wind a bit of teflon tape in the oring groove). If air is leaking from the trigger, replace the valve. If air is leaking out the bottom where the nail ejects, pull out the cylinger and lube the orings that seal the cylinder to the body of the gun.

Air leaking from top of the gun is caused by headvalve orings or trigger valve. Easy fix, remove the head and pop out the headvalve. Remove the orings from headvalve and clean grooves /orings. Reinstall orings and apply plenty of silicone paste lube. If guns holds pressure after cleaning and new silicone paste, order oring kit and have handy when problems start again. If the gun keeps leaking, remove the trigger valve and replace with new or change orings within valve. Good luck

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

There is an o ring that seals the valve that is operated by the trigger. It sounds like this oring is defective, or missing. Home depot or lowes should have no problem returning this for you if it is less than 90 days. it is fairly complicated to disassemble.

This series of nailers are the most difficult to repair. Your nailer has twice as many orings as the norm. Many have said that this nailer is over-engineered. There are three areas that could cause trigger area leak. To make it simple, lets do one area at a time. Use soapy water around the trigger area to find actual leak. If air is leaking from actual trigger valve (ms719-2) and its counterpart, the cartridge (ms719-1) then these two parts need to be removed. Check for scratches in the metal of the valve. If scratched try to remove with fine paper or replace as necessary. Most leaks are due to faulty trigger valve. If air is not leaking from actual trigger valve then remove the dump valve (mn120-5) at the back of the head of gun. There should be two small allen screws then metal lid, then spring and oring. Pull out dump valve and make sure all the orings are in order. This valve should be able to move, not too tight. Use silicone paste lube on all orings (available at Lowes in plumbing department, Danco brand used for faucet orings). Reinstall dump valve and related parts and check for leaks. If air seems to be leaking from bottom of gun neer trigger, most likely problem is cycle valve (mm419-1) and related vinyl seat. These two parts are in the valve housing under the trigger. This valve also moves within its housing and seats on a vinyl oring. This part could be clear vinyl or black rubber. New seats are clear. Inspect these parts for scratches/cuts and lube with silicone paste. Hopefully you would have solved the problem with trigger valve. Good luck with your repair and do not dispare, this is one of the most difficult guns.

Sounds like you need a repair kit or take to a tool company that installs the kit.