What is the primary function of spring brakes in a bus?

The primary function of spring brakes in a bus is to serve as a fail-safe mechanism that applies the brakes automatically in the event of a loss of air pressure in the braking system. When air pressure drops below a certain threshold, the spring brakes engage, which helps to safely bring the bus to a stop. This function is crucial for maintaining safety in case of a malfunction within the air brake system.

Understanding the role of spring brakes clarifies why the other options do not accurately define their purpose. For example, spring brakes do not provide additional power to the engine or assist in accelerating the vehicle, as these functions are unrelated to braking systems. They also do not only affect the rear wheels; instead, spring brakes can engage all wheels depending on the bus design, ensuring a balanced and effective stopping power in emergency situations. Lastly, while managing air pressure is part of the overall air brake system's function, it is not the primary role of spring brakes, which is specifically about applying the brakes.