What should you do to release parking brakes when preparing to test?

To safely release parking brakes when preparing to test, you should hold the brake pedal down. This action is paramount because it allows you to ensure that the vehicle remains under control as you disengage the brakes. By holding the brake pedal, you prevent any unintended movement of the vehicle, which is particularly important when transitioning from a stationary position.

Applying the brake pedal helps in managing the vehicle's stability and safely preparing for the next steps during the testing process. This practice aligns with safe driving protocols, ensuring that you maintain full control of the vehicle while making adjustments to the braking system.

In contrast, turning off the engine would likely engage the parking brakes or prevent access to the braking systems needed for such adjustments, and manually releasing the brakes without proper vehicular control can lead to unintended movement. Applying maximum pressure could unnecessarily stress the braking system, which is not a recommended procedure in this context.