What must parking brakes hold on air brake systems?

Parking brakes in air brake systems are designed to securely hold a loaded vehicle on an incline, ensuring safety and preventing unintended movement. The correct standard stated in federal regulations indicates that parking brakes must hold a fully loaded vehicle to its gross on a twenty percent grade. This requirement reflects the need for a robust braking system that can effectively engage and maintain control of heavier loads under sloped conditions.

This twenty percent grade translates to a significant slope, presenting a challenge for any vehicle that must remain stationary while parked. The braking system’s reliability in such situations is critical for operational safety, particularly in commercial and public transport scenarios like school buses. Therefore, achieving this standard ensures that the vehicle can be safely secured, regardless of the load conditions or terrain.

In contrast, the other options refer to grades that are either less steep or not in alignment with established standards for parking brakes, which do not meet the necessary criteria for safety and effectiveness.