At what pressure does the governor allow air to start pumping again?

The correct answer is 100 psi because in most air brake systems, the governor is designed to control the air pressure within the air tank. When the pressure in the tank drops below a certain threshold, typically around 100 psi, the governor will activate the compressor to start pumping air again. This process helps to maintain adequate air pressure in the brake system, ensuring that the brakes function effectively. Maintaining this pressure level is crucial for the safe operation of vehicles equipped with air brakes, as insufficient air pressure can compromise braking ability. The choice of 100 psi is based on standardized practices in air brake systems to balance the need for adequate braking power while avoiding over-pressurization that could damage components.