A system should not contain what type of gas before brazing to repair a leak?

In the context of brazing to repair a leak in an A/C system, the presence of refrigerant can be particularly hazardous. Brazing involves heating the metal parts to a high temperature in order to melt and flow a filler metal into the joint. If refrigerant is present in the system during this process, it can decompose when exposed to high temperatures, potentially producing harmful and toxic gases. Additionally, any remaining refrigerant can create pressure in the system, complicating the brazing operation and making it more risky.

In contrast, nitrogen is often used as a purge gas during brazing to prevent oxidation and contamination of the joint. It helps maintain an inert atmosphere around the area being brazed. Air is undesirable as well because it can introduce moisture and oxygen that may lead to oxidation. Ammonia, while not typically present in most residential or commercial systems, can also have complications if involved under high heat.

Therefore, ensuring that the system is free from refrigerant before undertaking any brazing work is essential for safety and to ensure a successful repair.