Final answer:
The complaint described in the question is most likely related to the governor of the air brake system.
Step-by-step explanation:
The complaint described in the question, which states that the air brake system won't pass air to the unloader to 'cut-off' the compressor, is most likely related to the governor. The governor is responsible for regulating the air pressure in the system and controlling the operation of the compressor. If the governor is not functioning properly, it may prevent the air from being passed to the unloader valve, resulting in the described issue.