In a normal operation, what does the primary air brake system control?

The primary air brake system is primarily designed to control the rear axle brakes in heavy vehicles, including school buses. This system is crucial for maintaining stability and control, especially during braking maneuvers. The separation of braking control between the primary and secondary systems helps to enhance safety by ensuring that if one system fails, the other can still perform adequately.

In most heavy-duty vehicles, the rear axle brakes are critical for stopping power due to their design and location relative to the vehicle’s weight distribution. Additionally, if problems arise in the braking system, the configuration helps to balance braking forces and prevent skidding or losing control during abrupt stops.

The other options relate to braking systems that may not primarily focus on the rear axle or encompass the entire wheel setup, which would involve the secondary air brake system or additional auxiliary systems. This specific focus of the primary air brake system on the rear axle is fundamental to maintaining effective braking performance in normal operating conditions.