What procedure should be followed to check the performance of the parking brake?

To effectively check the performance of the parking brake, reducing air pressure and observing if the spring brake activates is key. The spring brake, which is typically part of the air brake system in commercial vehicles, is designed to engage automatically in case of a loss of air pressure. This functionality is essential for safety, ensuring that the vehicle remains stationary when parked.

When the air pressure is reduced, if the spring brake properly activates, it indicates that the system is functioning correctly and capable of holding the vehicle in place. This procedure is a standard test to ensure the reliability of the parking brake under normal operating conditions.

The other choices do not align with the recommended practices for checking the performance of a parking brake. For instance, engaging the parking brake while driving can lead to potential vehicle control issues and does not provide a proper assessment of the brake’s effectiveness. Increasing engine speed while pressing the brakes or testing the brakes under load pertains more to checking service brakes rather than specifically evaluating the parking brake's performance.