I have a similar problem--I installed a new switch and suspected it is leaking. To confirm the leak, I swabbed some soapy water (bubble solution that our granddaughter plays with) around the area where new switch is screwed into the compressor tank. Sure enough, there are bubbles. Now I am trying to decide if I should try Teflon tape (as suggested above) or use a liquid sealant/adhesive on the threads (Porter Cable Helpdesk told me that the compressor was manufactured with a sealant on this switch, so per this forum's advice I successfully removed the old switch after applying heat with a heat gun. See other thread.

I hope this helps OP.