How can the holding power of trailer brakes be tested?

The holding power of trailer brakes is best tested by disconnecting the control line glad hands. When the glad hands are disconnected, the trailer's brakes can be engaged independently of the towing vehicle. This allows you to assess whether the trailer's brakes are functioning properly and can hold the trailer in place under various conditions, such as on an incline.

Testing the holding power this way ensures that the trailer's braking system is capable of adequately securing the trailer without relying on the towing vehicle's braking system. If the trailer rolls or exhibits inadequate holding power when the control line is disconnected, it indicates that the brake system needs maintenance or adjustment.

Other methods presented, like applying only the hand brake, observing brake fluid levels, or checking tire pressure, do not provide a reliable measure of the holding power of the trailer brakes themselves. These alternatives do not isolate the brake system's effectiveness and can lead to incomplete or misleading assessments of the braking capabilities.