If the system is diagnosed as short of refrigerant and has not been recently opened, there is probably a leak in the system. Adding refrigerant without first locating and repairing the leak, or replacing the faulty component, would not permanently correct the difficulty. The leak must be found.
Sufficient refrigerant may have escaped to make it impossible to leak test effectively. In such cases, add a 1/4-in. line-piercing valve to the compressor process tube. Add sufficient refrigerant to increase the pressure to 75 lb/in.2. Through this procedure, minute leaks are more easily detected before the refrigerant is discharged from the system and contaminates the surrounding air.
The line-piercing valve should be used for adding refrigerant and for test purposes only. It must be removed from the system after it has served its purpose. Braze a 4 in. piece of in. OD copper tube into the compressor process tube. Evacuate the system and recharge after repairs are completed.