Air conditioning works on high pressure only a small leak quickly causes a drop in pressure. It is often hard to locate air conditioning refrigerant leaks. There is a refrigerant that has dye in it that helps locate a leak. After recharging the air conditioning with the dye if the system leaks it leaves a stain to help locate the source of the leak. A "sniffer can also be used to locate leaks, it acts like a geiger counter as you locate the leak the "sniffer" beeps loudly.In either either case the system will need to be evacuated have the leak repaired and recharged with refrigerant and the proper lubricant.