What criteria must the parking brake meet to ensure vehicle safety?

The correct answer highlights that the parking brake must effectively hold the vehicle without being connected to the service brake system. This is a crucial safety criterion because the primary function of a parking brake is to secure the vehicle in place when it is parked, regardless of the operational condition of the service brakes, which are used for regular stopping while the vehicle is in motion.

This independence ensures that even if the service brakes fail (e.g., due to a leak in the hydraulic system or brake failure), the parking brake can still prevent the vehicle from rolling. Utilizing the parking brake in this manner is particularly important in situations where the vehicle is parked on an incline or slope, where the risk of movement is greater.

In contrast, any mention of the parking brake being connected to the service brake system undermines this safety aspect, as the vehicle would not be secured independently. The presence of oil or grease on linings contradicts functional effectiveness, as it can lead to poor braking performance. Likewise, the ability to disengage the parking brake only with manual force does not accurately reflect the standard mechanisms that often facilitate its disengagement via a lever or button, ensuring both usability and safety.