The braking power of the spring brakes depends on what condition?

• A. The service brakes being in adjustment
• B. The air pressure in the reservoir
• C. The vehicle's load
• D. The tire tread depth

Answer: (A)

The main idea here is how spring brakes actually produce braking power. Spring brakes are the parking/emergency brakes that apply by a set of springs pressing the brake shoes against the drums. How hard they can bite—their braking power—depends on whether the shoe-to-drum clearance is correct. That clearance is controlled by the service brake adjustment. If the service brakes are not properly adjusted, the shoes may not seat properly or the contact with the drum won’t be optimal when the springs try to apply, so the spring brakes can’t deliver their full braking force. The other factors listed—air pressure in the reservoir, the vehicle’s load, and tire tread depth—don’t directly determine how strong the spring brakes bite once activated. The air pressure is what releases or holds the springs, not the actual force they apply to the drum.