What must be done to test the trailer emergency brakes?

To effectively test the trailer emergency brakes, allowing air pressure to reach normal levels and then checking if the trailer rolls freely is essential. This process ensures that the emergency brakes are functioning properly. When air pressure in the braking system is at its normal operating level, it indicates that the brakes can adequately engage and hold. If the trailer rolls freely when you attempt to push it, it signifies that the emergency brakes have not engaged, thus highlighting a potential issue that needs to be addressed.

The other options involve aspects that do not directly facilitate the testing of the emergency brakes in a practical setting. For instance, checking the size of the braking system may provide information but does not evaluate the functionality of the brakes themselves. Consulting the vehicle manual can offer guidance specific to the trailer, but it doesn't replace the hands-on testing required to confirm proper brake function. Using a parking brake testing tool is not a standard procedure for assessing emergency brakes; it usually pertains to the testing of parking brake effectiveness. Therefore, allowing the air pressure to normalize and observing the trailer's movement is the most straightforward and reliable method to test the emergency brakes.