If either a metering valve, pressure differential valve, or proportioning valve is suspected of being defective, it should be ________

The most appropriate action when dealing with a suspected defect in a metering valve, pressure differential valve, or proportioning valve involves a thorough cleaning and inspection process. This ensures that the valve is free of debris, dirt, or any contaminants that could affect its functionality. Following the cleaning, the valve should be rebuilt if necessary, which often includes replacing any worn parts to restore proper operation. After rebuilding, a retest is essential to verify that the valve is functioning correctly and meets the required specifications.

Choosing "None of the choices are correct" does not provide a solution to the problem and overlooks the importance of maintaining and ensuring the operability of these critical components. In practical applications, addressing issues with valves by cleaning and performing maintenance is a standard and necessary procedure.