Presuming you’re referring to an aluminum line:
Locate the pinhole… score the area with 120 grit sandpaper. Then, Clean – Clean – Clean. Apply JB Weld, extending 1/2″ beyond the damaged area. Wait a few minutes until it starts to get tacky. Draw the refrigerant system into a slight vacuum, 10-12″ – this will pull the sealer into the hole. Draw a full vacuum after 1 hour. Allow JB Weld to cure for 2 days. Evacuate & recharge.

This is an approved repair procedure by appliance companies, and I’ve used it in Automotive applications countless times. Never fails.