I would start by running a vacum on the system it will tell you if you have a fairly large leak, but it willn't show you where the leak is. If the vacum test passes leave the system under a vacum and put about a half a pound of freon in the system. Then if you don't have a leak detecter spray all lines, condenser, accumulator, and allaround the compressor esp. Around where the clutch and lines connect on the back. The orfice tube is located right before the evaporator on the high side. It is where the high side becomes the low side. It is inside the line where the line comes apart right before the evaporator.