When should the parking brake be used in air brake vehicles?

Answer: B — Every time the vehicle is parked

The parking brake should be used every time the vehicle is parked to ensure it remains stationary and secure. This is crucial for safety; engaging the parking brake prevents the vehicle from rolling away, which is particularly important when parked on an incline or flat surface. Additionally, using the parking brake consistently helps to properly maintain the brake system and prevents the brakes from becoming inoperative over time due to rust or other issues. While it is particularly vital to use the parking brake on slopes, it is also essential on level ground to avoid any accidental movement caused by external forces like wind or when someone inadvertently bumps into the vehicle.