What is required for school buses equipped with air brakes?

For school buses equipped with air brakes, a low air pressure warning device is a critical requirement. This device serves an essential safety function by alerting the driver when the air pressure in the braking system drops to a level that may compromise the bus's ability to stop effectively. Air brake systems rely on compressed air to operate, and if the pressure falls below a certain threshold, it can impact the performance of the brakes significantly. Thus, having a warning system in place ensures that the driver can take appropriate action, such as finding a safe location to stop the bus and assessing the issue before it leads to a potential emergency situation.

The other choices involve systems or features that are not specifically mandated for buses using air brakes. For instance, while hydraulic braking systems are common in many vehicles, they are not used in air brake-equipped buses. Similarly, an automatic transmission is not a requirement connected to the braking system itself; it pertains more to the vehicle's engine and drivetrain. Lastly, a tire pressure monitoring system, although useful for general vehicle performance and safety, is also not a specific requirement associated with buses that have air brakes.