What method is used to test that the spring brakes engage automatically?

The method of fanning off air in the brakes until the pressure drops below a predetermined psi is the correct way to test that the spring brakes engage automatically. This procedure is essential because it simulates a loss of air pressure in the system, allowing the driver to verify that the spring brakes will activate when the air supply is insufficient. Typically, spring brakes are designed to engage if air pressure falls below a certain level, ensuring that the vehicle comes to a stop safely and prevents rolling away.

In this context, the importance of conducting this test is to ensure that the braking system is functioning correctly, which is critical for vehicle safety. Proper engagement of spring brakes under low-pressure conditions demonstrates that the braking system is reliable and can handle situations where air pressure might be lost.

Other methods mentioned would not effectively test the automatic engagement of the spring brakes. For instance, accelerating or relying on flat surfaces does not provide a reliable indication of how spring brakes will perform during an emergency situation or when there is a failure in the air system. Thus, fanning off the air provides a practical way to ensure that the spring brakes will engage as designed under low pressure.