After low gas pressure the aircon clutch relays are next suspect, they are a bit dodgy, either sticking closed and draining car battery or not closing at all. If you open the bonnet and shine a torch down at lowest pulley below alternator to will see that with aircon turned off the outer pulley still rotates with the drive belt but the centre of the pulley ( clutch plate ) is stationary, with air con turned on the solenoid should pull in the clutch plate which should rotate with the pulley, the solenoid pulls in clutch about every 10 seconds and then de-energises for a period ( this will be repeated as long as the AC is on and engine is running, the compressor does not run all the time - only when clutch engages, normally when clutch engages you will hear engine and aircon radiator cooling fans both run, and they stop when clutch drops out ).