The pressure sensor feeds a signal to the ECM. If there's not enough pressure or the sensor is stuck open, the ECM will not activate the AC relay.

You could remove the wiring harness from the pressure sensor and jump the wires. This fools the ECM into thinking there's enough pressure and activates the AC relay when the AC switch is turned ON. (Technicians have done this to intermittently activate the AC so they can recharged the system.)

BTW, there's no schrader valve at the pressure sensor. Remove it if you know for certain it's faulty.