How does a spring parking brake function?

A spring parking brake functions by utilizing a coil spring that applies the brakes through a service chamber. This mechanism is crucial because it allows the parking brake to apply pressure to the brake components without the need for hydraulic fluid or constant driver input. When the parking brake is engaged, the coil spring is compressed, and once released, it expands to apply the brakes firmly, securing the vehicle in place.

This system is particularly effective because of its fail-safe nature; even if air pressure is lost (in air brake systems), the spring action automatically secures the vehicle. The use of a coil spring ensures that the brakes are applied reliably, providing a strong hold when the vehicle is stationary.

The other options do not accurately describe the operation of a spring parking brake. For instance, hydraulic fluid is typically used in service brakes for normal stopping rather than in the parking brake mechanism. Additionally, while driver input is involved in engaging the brake, the spring mechanism operates independently once engaged, showing that it does not rely solely on driver input. Finally, the notion that the brake automatically engages when the vehicle is in motion is incorrect; the parking brake is designed to be engaged when the vehicle is stationary to prevent rolling.