Final answer:
Parking or emergency brakes in trucks and buses must be held on by mechanical force, typically through a spring-loaded system. These systems are independent of the hydraulic brakes and are essential for safety, ensuring that the vehicle remains stationary when parked.
Step-by-step explanation:
The question refers to the braking systems used in large vehicles such as trucks and buses. Parking or emergency brakes in these vehicles are critical for safety, ensuring that the vehicle remains stationary when parked, especially on inclines, and provides an additional stopping mechanism in case of the primary brake system failure.
Parking or emergency brakes on trucks and buses are typically held on by mechanical force, not hydraulic pressure as is the case with the main braking system. This mechanical force is often provided by a spring-loaded system, which is engaged by a lever or a pedal in the driver's cab. The parking brake system functions independently of the vehicle's hydraulic brakes, and it is designed to stay engaged until manually released by the operator, ensuring that the vehicle cannot roll away when left unattended. This system acts on the brake shoes or pads to apply a holding force to the wheels directly.
In addition to their essential role in vehicle safety, there are regulatory requirements governing these braking systems. In the United States, for example, the Federal Motor Carrier Safety Administration (FMCSA) sets standards for parking brake systems on commercial vehicles, mandating that these systems must be capable of holding the vehicle stationary on a certain percentage grade. These standards are enforced to ensure that all commercial trucks and buses operate with a reliable parking brake system. Operators of these vehicles are required to understand and regularly test their parking brakes as part of routine pre-trip inspections.