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.
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.
If the check valve is now.
How to fix it.
You can try to clean out the old one.
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.
Replace the safety valve if it leaks air constantly.
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 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 air leaks out the unloader when the pump stops replace the check valve because the check valve leaks air back towards the unloader.
Either one will fit fine though the dewalt porter cable unit will give you slightly higher ground clearance.
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.