What triggers the governor to start pumping air again?

The governor is a critical component of an air brake system, controlling the air pressure within the brake system. When the air pressure in the system drops below a certain threshold, the governor activates the air compressor to start pumping air again. This low-pressure activation point is known as the "cut-in" level.

Reaching the cut-in level around 100 psi is the correct answer because most air brake systems are designed to replenish air pressure once it falls to this point. This ensures that there is sufficient air pressure available to operate the brakes effectively. The governor will stop the compressor from running once the system reaches a predefined level of pressure, usually around 125 psi, allowing for proper brake operation and preventing over-pressurization.

Understanding this mechanism is crucial for maintaining the safety and reliability of the air brake system in commercial vehicles, which rely heavily on adequate air pressure for effective braking performance.