Illinois CDL Practice Exam.

The correct answer is D. The purpose of the air compressor governor in an air braking system is to prevent excessive air pressure in the system. The governor monitors the air pressure in the system and signals the compressor to stop compressing air when the maximum pressure is reached. This helps ensure that the air pressure within the system remains at a safe and optimal level for proper brake operation. Options A, B, and C are incorrect: A. To control air tank drains: The air tank drains are typically controlled by separate valves or mechanisms and are not directly related to the air compressor governor. B. To operate the foundation brakes: The foundation brakes are operated by applying air pressure to the brake chambers, which is controlled by the brake pedal and relay valves, not the air compressor governor. C. To pump air into the air storage tanks: The air compressor is responsible for pumping air into the air storage tanks, not the air compressor governor.