What position must the retaining valves be in during an initial terminal and road air brake test?

During an initial terminal and road air brake test, the retaining valves must be in the exhaust position. This is crucial as the retaining valves are designed to retain air in the brake cylinders to maintain the braking force while the train is in motion. However, during testing, it is essential that these valves allow air to escape, ensuring that all components of the brake system are properly tested and functioning. The exhaust position removes any resistance from the retaining valves, allowing for a comprehensive evaluation of the air brake system's performance and confirming that it operates correctly without any interference from the retaining valve settings.

In this context, other positions such as active or release would not facilitate the required conditions for an effective brake test, as they either retain air in the system or alter the brake function in ways that could conceal potential issues within the air brake circuit.