Edited: 9/30/2007 6:24:28 PM by Cruz Lo
Use the pressure switch to ground the relay coil.
Relay coil ground > pressure switch > (other wire of pressure switch to ground.
This way when the pressure switch is made the relay makes, then the compressors come on.
BUT I would also have a regular switch in the circuit or a switch for the power to the relay coil to manualy shut the system down so if you have a leak you won't run your battery down.
Good luck.
Oh you can also search this forum for more information. Sucicide doors web site also has diagrams.
http://www.suicidedoors.com/how_to/wire-diagrams/compressor.gif