In my OBA setup I out in a 1/4 Tee for the pressure switch. The compressor is running on a Viair 110-145 PSI switch, with a second switch, a 90-120 on the other port. The key switched power feeds both switches, the 145 goes to the relays for the compressors, the 120 goes to a 12 volt radio shack buzzer under the dash. If the tank ever drops under 90 psi, the buzzer sounds, and shuts up at 120. Normally the compressors kick on at 110, so I only hear the buzzer if I either haven't run the truck in a week, or we have been fillings things (like pool toys when camping) with the engine off. My air tank is rated to 150, which is why I run the 145 switch. I replaced the drain plug with a 150 psi blow off valve.
