Unloader valve only leaks while compressor is running?
Joe, the air compressor unloader valve is supposed to open when the air compressor reaches the cut out pressure level and the motor stops.
When the compressor starts again, the unloader valve is supposed to close, to ensure that the compressed air that is being generated by the air compressor pump is directed into the tank.
You don’t indicate what your compressor is, or what the pressure switch / unloader valve combination is on your air compressor making it harder to diagnose the problem and solution.
In the photo above you can see the pin that protrudes from the side of the pressure switch. It is this pin that toggles this unloader valve.
So, first you have to make sure that the unloader valve is actually opening and closing by monitoring it while the air compressor runs up to cut out pressure and stops.
Then, if the unloader is actually working, the problem is probably a leaking unloader valve.
Dismantle it, clean it, reassemble it, and test, or just replace it.