Have you made any other changes or additions to your system? Is thebpressute switch going to a relay? It is a simple positive In Out switch. Run a got wire from the battery to the pressure switch to the compressor and isolate it from the rest of the car and test. If it turns off it is your loom, if it runs on then the new switch is knackered too. Is the pressure switch bolted to the compressor? Change ports.

In fact...... change ports first. Bet that is your trouble.