What are two primary functions of the spring parking brakes in a dual air brake system?

The spring parking brakes in a dual air brake system serve two primary functions: acting as a parking brake and functioning as an emergency braking system.

As a parking brake, these brakes are designed to keep the vehicle stationary when it is parked, providing the necessary hold to prevent movement on inclines or declines. This is particularly crucial for large vehicles that can easily roll if left unattended.

In addition to their parking function, spring brakes also engage as an emergency braking system when there is a loss of air pressure. In a dual air brake system, if the primary air supply fails, spring brakes automatically activate to bring the vehicle to a stop, ensuring safety.

The other options do not accurately represent the roles spring parking brakes fulfill; acceleration and deceleration relate to dynamic vehicle control rather than holding functions, while torque converters and transmission control, as well as steering control and alignment, operate in completely different areas of vehicle mechanics unrelated to the braking system.