Under normal conditions, what must a driver do first to engage the parking brakes?

To engage the parking brakes under normal conditions, a driver must first let the air out of the spring brakes. This process involves allowing the air pressure in the brake system to drop, which activates the spring brakes that hold the vehicle in place when it is parked.

Spring brakes are a crucial safety feature in air brake systems, designed to engage automatically when the air pressure in the system falls below a certain level. By releasing the service brakes, the driver ensures that the vehicle does not roll while transitioning to engage the parking brakes. Thus, managing air pressure within the system is a vital step in activating the parking brakes effectively.

The other options, such as pressing the brake pedal or turning off the engine, do not directly relate to the process of engaging the parking brakes. While releasing the service brakes may seem like a necessary action, it is not the first crucial step that allows the spring brakes to engage properly when parking the vehicle.