What are spring brakes?

Answer: B — They serve as a backup braking system for air brakes

Spring brakes serve as a backup braking system for air brakes. In vehicles equipped with air brake systems, spring brakes are engaged automatically when the air pressure drops to a certain level, typically indicating a failure in the primary braking system. This function is crucial for safety, as it ensures that the vehicle can stop even in the event of a loss of air pressure, thereby preventing runaway conditions. The spring brakes are also used to hold the vehicle stationary when parked, but their primary role as a backup system is what distinguishes them in the context of braking safety. They utilize powerful springs that can apply enough force to bring the vehicle to a stop, aiding in controlling it during emergencies. While enhancing fuel efficiency and serving purely as a parking mechanism are additional considerations for brake systems overall, they do not capture the core function of spring brakes in relation to their operational role alongside the primary air brake system.