Which of the following best describes spring brakes?

Spring brakes are a crucial component of a vehicle's braking system, particularly for large commercial vehicles. These brakes are designed to provide additional safety and stopping power. The correct description states that spring brakes are held back by powerful springs and air pressure.

In this system, powerful springs are used to apply the brakes when the air pressure in the brake system drops below a certain level. This design is particularly important in ensuring that the brakes engage automatically in the event of a loss of air pressure, preventing the vehicle from rolling unintentionally and providing a fail-safe mechanism.

The other descriptions fall short in accurately capturing the function of spring brakes. For example, while hydraulic brakes rely on fluid to operate, spring brakes employ a combination of mechanical springs and air pressure, not hydraulic fluid. Additionally, spring brakes do not require manual adjustment as part of their design; they are self-adjusting in response to air pressure. Lastly, stating that spring brakes are used solely for downhill driving misrepresents their intended purpose, as they function primarily as a parking brake and can be used in various situations to enhance braking performance and safety.