Q: Refrigerant leak caused by screw tightened too tightly

Last summer, the air conditioner wasn’t cooling, so the dealership filled the unit with refrigerant. This summer I discovered that the refrigerant had leaked out again! Back to the dealership and this time they added a die when they refilled the unit so that they could find the leak. This is what they found: when the car was built, the screw at the top was tightened too tight and that had caused a small leak which was allowing the refrigerant to leak out. Since this happened when this car was manufactured, I feel they should stand behind both the part and labor, even if it is an older model. They only offered to provide the part. Since this was a manufacturer error, do you know how I can get them to stand behind the labor too?