I got the pressure switch off ebay
http://www.ebay.com.au/itm/VIAIR-90-...item337cfa474a
The way it works is like this (Keep the switch on the actual pump on)
Initial condition -there is zero air pressure and rocker switch is off
1 - Turn the rocker switch on and your motor (or accessories) on. Power runs from accessories through the rocker switch, through the pressure switch contacts, through the relay and energises the relay, thus closing relay contacts and allowing power through compressor.
2- When the pressure in the system reaches the 'off' pressure on the pressure switch (int the case of the one I have shown in link - 120psi), the contact on pressure switch opens, causing the relay to de-energise, opening the relay contact and stopping power to the compressor.
3- As you use the air, the pressure drops to below the 'on' setting (in the case of the one I have shown in link - 90psi), the contact on presure switch closes again and the relay is energised again turning compressor back on.
Use an inflator with a gauge like this one to pump the tyres up.
http://www.supercheapauto.com.au/onl...d=293784#Cross
Also, make sure you use a relay and don't just use the pressure switch to supply the power. Those pressure switches are only rated at 20A