What is the procedure to test the trailer emergency brakes?

To test the trailer emergency brakes effectively, the correct procedure involves charging the air brake system and then confirming that the trailer rolls freely. This step is essential because a properly functioning emergency brake system should allow the trailer to be securely engaged and then to verify that it will release and allow for free movement when the brakes are not applied.

When the air brake system is charged, it ensures that there is adequate pressure in the system to operate the brakes properly. After charging, checking that the trailer rolls freely confirms that the spring brakes are not engaged, which is an important safety check. This procedure helps ensure that the emergency brake system is operational and will act appropriately in case of an emergency.

The other choices might refer to various aspects of brake system maintenance or testing but do not describe the specific procedure for testing the emergency brakes. For instance, applying the parking brakes without first charging the system may not adequately assess the functionality of the emergency brakes. Measuring pressure in the brake lines might indicate system health but does not specifically evaluate the functionality of the emergency brakes. Observing the ABS light is important for assessing the anti-lock system's functionality but does not provide information about the emergency brake's effectiveness.