How does the manual say a driver should test that a trailer's emergency brakes will actually engage if the air supply is lost?

Explanation

To test the trailer emergency brakes, the manual has the driver first charge the air brake system and confirm the trailer rolls freely, then move the trailer air supply control to the emergency position, pulling the knob out, which simulates a loss of air supply; a gentle pull with the tractor then shows whether the emergency brakes have actually set and are holding. This is manual-only procedure, topic-scoped to 49 CFR 383.111(c)(2). Checking only the ABS lamp tests an unrelated electronic indicator rather than actual mechanical brake engagement. Disconnecting the emergency line and driving is an unsafe, untaught way to test emergency brakes that risks losing control if they fail. Assuming one brake system's function guarantees another's is wrong, since the service brakes and the spring-actuated emergency brakes are mechanically distinct systems that must each be verified.

Source: 49 CFR 383.111(c)(2)