Which element is essential for preventing air from flowing back in the brake system?

Answer: C — Check valve

The essential element for preventing air from flowing back in the brake system is the check valve. This device is designed to allow air to flow in one direction while stopping any reverse flow. In a braking system, it ensures that once air has been supplied to the brake cylinders for actuation, it cannot return to the compressor or reservoir, maintaining the necessary pressure for effective braking. This is crucial for maintaining consistent brake performance and safety. While retaining valves and brake cylinders play important roles in the overall braking system, they do not specifically serve the function of preventing backflow in the same manner that check valves do. A pressure gauge, on the other hand, is used primarily to monitor system pressure but does not have the function of controlling airflow. Thus, the check valve is vital for ensuring the integrity of the air brake system by preventing unwanted reversion of air, making it the correct answer to the question.