Final answer:
Technicians should be careful when brazing lines to evaporator coils that have pre-installed thermostatic expansion valves because the heat from brazing can damage the expansion valve.
Step-by-step explanation:
Technicians should be careful when brazing lines to evaporator coils that have pre-installed thermostatic expansion valves because the heat from brazing the lines can cause damage to the expansion valve. When the lines are brazed, the heat can cause the working fluid, which is a gas, to condense into a liquid. If the expansion valve is not protected or removed during brazing, the heat can damage its sensitive components or cause it to malfunction. This can result in poor cooling performance or even system failure.