What happens to the parking brake knob when the low air pressure warning activates?

When the low air pressure warning activates, the parking brake knob is designed to pop out. This mechanism serves as a crucial safety feature in commercial vehicles equipped with air brakes. When air pressure drops below a certain threshold, it indicates that the braking system may not function properly. Consequently, the brake system engages to prevent the vehicle from moving unintentionally, and the parking brake knob pops out to signal that the brakes are applied and the vehicle should not be driven.

This physiological response helps drivers immediately recognize that they should not attempt to operate the vehicle until they address the low air pressure condition. It's essential for ensuring the driver's awareness and maintaining safety on the road, as driving with insufficient air pressure can compromise braking effectiveness.