Unplug the compressor drain all the air from the tank remove and clean or replace the tank check valve.
Compressor drain valve leaking air.
If you aren t sure about what that is have a read of the unloader valve page on this site.
The safety valve pops open to relieve excessive air pressure if the pressure switch fails to shut off the air compressor at the cut out pressure setting.
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.
You can try to clean out the old one.
How to fix it.
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.
If you feel any air leak around or beside the pressure switch it s a pretty good bet that your tank check valve is leaking.
You can either buy bostitch ball valve compressor ball type drain valve.
You can take a pipe cleaner and run up in there with it fully open and see if you can get anything to come out of it.
If the check valve is now.
Replace the safety valve if it leaks air constantly.
Or buy dewalt porter cable compressor oem replacement ball valve n072023.