What must never be done to a copper joint when the system is under pressure?

A copper joint should never be soldered or brazed when the system is under pressure due to safety and integrity concerns. Soldering and brazing involve heating the joint to a temperature sufficient to melt the solder (or filler metal) and form a bond. If performed while the system is pressurized, it can lead to dangerous situations such as the risk of explosions or the release of refrigerants, as the heat can cause the refrigerant to expand rapidly. Furthermore, applying heat to a pressurized system can potentially weaken the joint or cause it to fail, leading to leaks or catastrophic failures.

For context, cutting or welding a joint would typically not be performed under pressure either, as it poses significant risks; however, the focus here is specifically on the application of heat, which is critical in soldering and brazing. Tapping or drilling into a copper joint under pressure would also be inadvisable, but such actions typically do not require heat and are less commonly related to joint integrity. Repairing a joint under pressure can involve methods that do not necessitate heat, making soldering and brazing distinctly more hazardous in this context.