What should you do after releasing the parking brake to check spring brakes?

After releasing the parking brake to check the spring brakes, the correct action is to shut off the engine. This is because you want to ensure that the spring brakes are functioning correctly without the influence of the engine's power. By turning off the engine, you eliminate any pressure from the air brake system that could affect the operation of the spring brakes, allowing you to observe their release and engagement accurately.

In this scenario, the other actions—like starting the engine, pumping the brakes, or setting the transmission to neutral—would not provide the necessary conditions to check the spring brakes effectively. Starting the engine may introduce additional air pressure, influencing the brake operation, while pumping the brakes could interfere with the checking process, and setting the transmission to neutral does not pertain directly to verifying the functioning of the spring brakes.