Which brakes does the secondary air brake system typically control?

The secondary air brake system is primarily designed to provide redundancy and maintain safety in case of a primary system failure. It typically engages the rear axle brakes. This system ensures that the vehicle can still be stopped effectively if the primary braking system encounters issues.

In most vehicles, the primary brakes control the front axle, while the secondary system is responsible for the rear axle. This setup allows for improved braking efficiency and stability, particularly under load, as the rear brakes can handle more weight and assist in stopping the vehicle safely.

The other options, such as the parking brakes and trailer brakes, serve different purposes. Parking brakes are utilized for securing the vehicle when stationary and operate separately from the primary and secondary brake systems. Trailer brakes are designed to engage brakes on a connected trailer, which is not a part of the standard air brake system's configuration. Thus, the secondary air brake system’s focus on managing the rear axle brakes is essential for maintaining optimal vehicle control and safety.