You can either buy bostitch ball valve compressor ball type drain valve.
Compressor drain valve leaking air.
Also i would take a pipe cleaner saturated with oil and run around the inside of the drain valve and the screw portion of it.
If the check valve is now.
Or buy dewalt porter cable compressor oem replacement ball valve n072023.
1 unplug the compressor 2 drain all the air from the tank 3 remove the lines connected to the tank check valve 4 remove the tank check valve inspect clean re install the same one or install a new one 5 reassemble and test the compressor operation.
You can try to clean out the old one.
If air continues to leak out of this vent after the compressor stops then it s coming from your tank and that means that the check valve that is supposed to keep air in the tank when the compressor stops is leaking.
Replace the safety valve if it leaks air constantly.
Unplug the compressor drain all the air from the tank remove and clean or replace the tank check valve.
Either one will fit fine though the dewalt porter cable unit will give you slightly higher ground clearance.
If you aren t sure about what that is have a read of the unloader valve page on this site.
How to fix it.
If air leaks out the unloader when the pump stops replace the check valve because the check valve leaks air back towards the unloader.