When it is necessary to add refrigerant to a system after a loss it indicates that there is a leak in the system. The leak must be located, repaired and the system leak tested. If the leak is found to be on the low side of the system the repair can be made after the system has been ‘pumped down’ to a balance in pressure. Do not draw in any air which contains moisture, especially where hygroscopic ester oils are concerned, (for example R134a). Some oils are not affected and accept the ingress of air and moisture, recovering after a vacuum is drawn with a recovery unit.