What type of pressure must the parking or emergency brake on a heavy vehicle be held by?

The parking or emergency brake on a heavy vehicle must be held by spring pressure. This design is critical for ensuring the safety and reliability of the braking system. When the brakes are engaged, strong springs apply the necessary force to keep the brake pads in contact with the brake drum or disc. This mechanical action ensures that the vehicle remains stationary even on inclines or uneven surfaces.

Using spring pressure for the parking or emergency brake is a safety feature designed for fail-safe operation. In the event of a loss of air or hydraulic pressure, the brakes will still engage due to the inherent tension of the springs, preventing the vehicle from rolling unexpectedly.

Other systems like air pressure and hydraulic pressure are typically used for the service brakes, where the brakes engage while the vehicle is in motion. Electrical pressure is not utilized for braking in heavy vehicles, so it is not relevant in this context. Understanding the role of spring pressure helps emphasize the critical nature of emergency and parking brake systems in heavy vehicle operation.