After cleaning replace the parts.
Compressor drain valve leaking.
They usually use a high strength thread locker on the drain valves so it may just be very very hard to remove.
If the check valve is now.
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.
How to fix it.
I would recommend replacing the valve and using some thread locker when you install the new one.
Unloader valve is leaking air when the air compressor reaches the normal cut out pressure and stops but you notice while the air compressor is off that air is leaking out of a valve attached to or perhaps inside of the air compressor pressure switch.
Product links below in order of appearance.
Turn off and unplug the air compressor.
The valve on the outside of the pressure switch may look something like the one in the image.
To fix the air leak caused by a faulty drain valve there are three choices.
You can try to clean out the old one.
Drain the air tank from the drain valve.
By the way c clip ring shown in the picture prevents the screw from being removed from the drain valve assembly.
Which i ve never seen done on any compressor tank.
Remove the regulator body from the manifold with a wrench.
Makita mac700 air compressor.
Http amzn to 2ir0elq pittsburgh forge 12 piece screw extractor.