At what pressure should the stoplight switch illuminate in air brake systems?

The stoplight switch in air brake systems is designed to illuminate when the air pressure drops to a critical level, indicating a potential failure or safety concern. The correct answer indicates that the switch should activate at 6 psi or less. This threshold is established to give adequate warning to the driver before air pressure falls too low for safe operation of the vehicle.

At this pressure level, the stoplight serves as an alert, prompting the driver to take immediate action, such as reducing speed and seeking a safe location to stop, due to the risk of losing braking ability.

Using 6 psi as the trigger point ensures that drivers are made aware of diminishing air pressure before reaching a point where the vehicle can no longer safely stop. This safety feature is crucial for maintaining control over the bus and ensuring the safety of passengers and other road users.