If the governor valve fails to "unload" the compressor, what will protect the reservoirs from over-pressurization?

Answer: B — safety valve

The safety valve is a crucial component designed to protect air brake systems by preventing over-pressurization of the reservoirs. If the governor valve fails to unload the compressor, the compressor could continue to generate air pressure beyond the safe operating limits. In such a scenario, the safety valve acts as a fail-safe mechanism. It is calibrated to open at a predetermined pressure, allowing excess air to escape and thus preventing the pressure in the reservoirs from exceeding safe levels. Unlike the safety valve, the compressor belt is simply a component that drives the compressor and does not play a role in pressure regulation. An air dryer is responsible for removing moisture from the compressed air but does not offer protection against over-pressurization. A pressure gauge, while useful for monitoring current pressure levels, does not provide any protective function if the pressure exceeds the safe limit. Therefore, the safety valve is the only component among the options that directly ensures the safety and integrity of the air brake system in the event of a governor valve failure.