Volvo Repair: S60 aircon, liquid freon, volvo s60

Question
Volvo S60 2002 England. I had a schrader valve leak and so had aircon re-charged.  Ok for a couple of months but now has developed a constant hissing noise from dashboard when aircon switched on.  Also, the guy said it required about one third of a litre of oil in with coolant but only aded 33cc - on tenth of that. Can this be related? How much oil is needed?  Thanking you in advance.

Answer
  Keith, what you are probably hearing is the liquid freon turning into gas in the evaporator. The charge is probably incorrect. At this point. I would fully reclaim the system, flush it to remove all oil, and start over with the correct oil and freon charges. It will not be cheap but otherwise you are just guessing. Have it double checked for leaks before evacuating. I always add dye when I charge a system for future ease of leak detection.
   Is it cooling well?

Mark