The Nason pressure switch is a normally closed switch, which means it will normally pull your digital input to 0, except once it reaches 120psi, it will switch to 1. All you do to run the compressor knowing that, is set whichever relay you have it connected to, to the opposite of the input.
In your case, I believe this is:
Code:
relay7_fwd = !rc_dig_in02;