Final answer:
The parking or emergency brakes on heavy vehicles are typically held in position by a mechanical system, such as springs or a manual rod, as a safety measure to ensure the vehicle remains stationary when parked. These systems are designed to be fail-safe, engaging the brakes automatically in case of air pressure loss.
Step-by-step explanation:
The question concerns the brake systems used in heavy vehicles. Specifically, it addresses the type of mechanism that can reliably hold the parking or emergency brakes in the engaged position. In heavy vehicles, the parking or emergency brakes are typically held in position by mechanical means, such as springs or a manual push/pull rod, and not by hydraulic or air pressure which can potentially leak away.
The reason for this is safety; a mechanical system provides a fail-safe position in case other systems fail, ensuring that the vehicle remains stationary when parked.
Most heavy trucks use a spring-applied, air-released brake system for parking. The spring creates a constant force that applies the brake, and the brake is released by increasing air pressure to compress the spring. This design ensures that if there is a loss of air pressure, the springs will automatically apply the brakes, preventing the vehicle from rolling or shifting unexpectedly. While this system is highly reliable, it is also crucial for drivers to perform regular maintenance checks to ensure that all components, including springs and mechanical linkages, are in good working condition.
It is also worth noting that regulations require heavy vehicles to have parking brakes that are capable of holding the vehicle stationary on an incline, a clear testament to the importance of a robust mechanical braking system. The design of these systems adheres to strict safety standards to prevent accidents that could arise from a failure in the braking system.