What action should be performed after coupling the brake pipe hoses when adding helpers?

Opening the locomotive's brake pipe angle cock first is essential after coupling the brake pipe hoses when adding helpers because it allows air to flow into the brake pipe of the newly coupled locomotives. This action ensures that the brake system is connected properly and that the air pressure can stabilize across all connected units. By doing this first, you help integrate the new locomotives into the existing air brake system more effectively, allowing for coordinated operation.

Once the angle cock is opened, air will begin to fill the brake pipe, ensuring that all locomotives are on the same pressure level, which is crucial for safe and effective brake operation. This sequence helps to prevent issues that could arise from uneven air pressure among the units.

In contrast, removing air from the system, testing for leaks, or closing the throttle would come after opening the angle cock. These actions are part of the overall process, but the initial step is to ensure that the system is connected properly by opening the angle cock, allowing for a consistent air supply throughout the train.