My D51257 nailer will not fire, and air escapes from the trigger. Do I need a new trigger valve? If so, how do I replace it?

debris or damaged trigger

Sounds like your gun needs servicing. normally strip, clean and usually fit new O rings to head valve assy, piston & usually sleeve.

Remove the nails and take off the trigger to expose the trigger valve (usually part # tva6). Connect air and spray soapy water on the valve to determine if the air is leaking from the center of the valve or around the outer edge between the body and valve. If leaking from the center, could be defective valve or debris under oring. Pop out the valve and examine internal parts for cut oring or defect. If leaking from between body, you could have cut orings installing the valve. Make sure that there are no rough edges in the body of the gun. Bump fire or single fire happens with the trigger type not the valve. Bump fire trigger is usually black metal and single fire trigger is light color plastic. Good luck

If the new trigger valve leaks air when you connect the air supply, most likely defective valve or air is leaking past outer trigger valve orings due to debris in the slot or new orings were cut when the valve was pushed in. If air leaks when you depress safety to fire gun then most likely problem is in the head valve or safety foot is not pushing the firing pin all the way in. Easy test, remove nails and remove the trigger to expose the firing pin. Connect air and very quickly depress the firing pin all the way in. If the gun fires, adjust the safety foot as needed. If the gun does not fire, remove the head and check headvalve orings for good snug fit against the cylinder within the head. The surface were orings make contact must be very smooth and not too deeply worn. If you notice fine scratches remove with fine sandpaper. Reinstall head using plenty of white grease on all the orings within the head and retest. Good luck

Check the safety trip make sure it is pushing back far enough. If it doesn't then it won't do anything. When it was fine just moments earlier.

depends on where air is escaping from. If it is around the trigger then you need a new trigger valve (I have detailed instructions in the tip section on repair) If it is leaking thru the cap you have a pinch o ring or stuck firing valve. Let me know exactly where air is leaking and I will try to pinpoint the problem for you.

As the contact arm and trigger inner lever wear, the trigger will not push the trigger valve pin all the way in. The pin on the trigger valve also wears. One deficiency or a combination the three problem areas is causing the problem. Remove the trigger and look carefully at the inner lever (thin metal part ). If it looks rather worn and the trigger itself fits loose on the retaining pin, replace with new. Replace the trigger valve if the center pin seems loose in the valve and looks like it has worn shorter. ;Good luck.

There are about three issues with this gun that could cause your problem. The safety contact arm not depressing the trigger pin fully, worn trigger valve (tva6), or leaking orings in the headvalve. Easy tests. Remove the nails, and remove the trigger to expose the tva6 valve. Reconnect air and with very quick and firm action, depress trigger pin all the way in. If the gun fires, adjust safety foot and trigger so that the trigger pin is depressed all the way in each time that you fire the gun. If the gun does not fire, remove the head and remove the headvalve. Examine the surface/s where Orings make contact with magnify glass looking for fine scratches. Sand with 400 grit paper. Orings must make good contact here or gun will not fire. Lube orings and surface with silicone paste lube (sold at Lowes in the plumbing department or swimming pool supply store). If no fine scratches and surface looks smooth, replace Orings in head and the tva6 valve. Good luck with your repair/

you need a new trigger valve assembly, there is an o ring that has split. Contact Bostitch for a replacement part.

could be either that or a damaged o ring, when was the last time serviced, all o rings replaced?? could be that the tool needs a thorough cleaning and new one installed. it is not hard