Final answer:
The governor starts the air compressor in a pneumatic system when the pressure reaches the cut in level, typically around 100 psi for many truck air brake systems.
Step-by-step explanation:
The question pertains to the workings of an air governor in a pneumatic system, often used in vehicles like trucks for air brake systems. The governor regulates air pressure by restarting the compressor when the system's pressure falls to a predetermined 'cut in' level. The correct 'cut in' pressure level is specific to the system's design and usage, but for many truck air brake systems, this pressure is commonly around 100 psi (B), although it can vary between different vehicles and systems.