Volvo Repair: S70 Air conditioning, refrigerant, sniffer

Question
My wife's 2000 S70 AC (145,000) is out.  If you add a refrigerant, it will work for awhile, but will indicate overcharged.  I have gauges and refrigerant, but no data.  How much (in PSI) does this system take, and is there some other troubleshooting that I can do?  I also have a "sniffer" for this type refrigerant.  

Answer
Nathan, The system hold 1.63 lbs of refirgerant and its very picky.  at 2000 rpm your PSI should be at about 28-30 PSI.  At rest at around 40-42.  You should be doing this from the low side port.  You may be experiecing at clutch that needs reshimming.  Common issue on these cars.  Hope this helped.  Roger