What keeps the powerful springs of spring brakes (emergency brakes) held back?

The correct answer is that air pressure keeps the powerful springs of spring brakes, also known as emergency brakes, held back. In air brake systems, which are commonly used in large vehicles like school buses, the spring brakes are engaged by means of a strong spring mechanism. When the vehicle's air brake system is pressurized, the air pressure holds these springs in a released position, allowing the vehicle to move freely.

In the event of a loss of air pressure, such as during an air system failure or when the driver actively engages the spring brake, the air pressure that was keeping the springs retracted is released. This results in the springs expanding and applying the brakes automatically, bringing the vehicle to a stop. This design ensures that in case of an emergency or failure of the primary braking system, the vehicle will still be able to stop, enhancing safety.

Other mechanisms, such as hydraulic pressure, mechanical resistance, or electrical current, do not play a role in holding back the spring brakes in this specific air brake context, which is why they are not the correct choices for this question.