When performing a trailer brake test, what should be engaged?

In a trailer brake test, the correct action is to engage the red valve only. The red valve controls the trailer brakes, and by engaging it, you ensure that the trailer's braking system is activated for testing purposes. This test is critical to confirm that the trailer's brakes are functioning properly and can effectively slow down or stop the trailer in coordination with the truck's brakes.

Engaging the yellow valve, which controls the emergency brakes, is not necessary during the trailer brake test and can lead to confusion about the operational status of the trailer brakes. It's important to isolate the test to the trailer's braking systems to accurately assess their performance.

The process of engaging only the red valve while performing the test also allows you to check for any leaks or malfunctions specifically within the trailer's braking system without engaging the emergency features that are not part of the intended test scenario.