When the air pressure falls below the minimum allowed in the system, what does the governor do?

When the air pressure in the system falls below the minimum allowed, the governor's primary function is to prevent more air from escaping the air tanks. This mechanism is crucial for maintaining adequate air pressure since the air brake system relies on a certain pressure threshold to function properly.

By preventing further air loss, the governor helps ensure that the system can recover pressure as needed, allowing the compressor to replenish the air in the tanks once the pressure rises again. This action is vital for maintaining operational safety; if the air pressure continues to fall without regulation, it could lead to brake failure, posing a significant risk while driving.

In contrast, other options do not accurately reflect the function of the governor. The increase of air pressure in the tires does not fall under the governor's responsibilities, nor does the activation of the emergency brake or the shutdown of the compressor occur when pressure drops. The governor specifically focuses on regulating air loss to keep the braking system functional.