Which of the following is NOT considered a brake sub-system?

The parking brake system is typically not classified as a sub-system of the primary braking system in most contexts. Instead, it serves as a secondary means of securing a vehicle when it is stationary. The primary function of the parking brake is to prevent the vehicle from rolling when parked, allowing for safety features that can aid in securing a vehicle on an incline, for instance.

In contrast, the apply system, balance system, and boost system are integral parts of the primary braking mechanism. The apply system is essential for translating the driver's input into braking action on the wheels, the balance system is crucial for ensuring that braking forces are distributed effectively among the wheels, and the boost system enhances brake performance, making it easier for the driver to exert the necessary force to slow down or stop the vehicle.

Understanding these distinctions highlights the role of the parking brake as a supportive mechanism rather than a central component of the braking sub-systems.