The parking or emergency brake on a heavy vehicle can only be held in position by something that cannot leak away, like:

The parking or emergency brake on a heavy vehicle relies on a reliable source of pressure to maintain its position. Spring pressure is the only method among the options given that does not risk leaking away. In air brake systems, for example, the parking brake is often activated by a powerful spring mechanism. When the brake is engaged, the spring is compressed and holds the brake in a locked position.

This is crucial for safety because if a brake were to be maintained by pneumatic or hydraulic pressure, a leak could lead to brake failure. Pneumatic pressure in air systems and hydraulic pressure in hydraulic brake systems can dissipate, making them unreliable for long-term holding purposes. Mechanical tension could also be affected by wear or other mechanical issues and might not provide consistent holding power.

Using spring pressure ensures that the brake remains engaged even in the event of a failure of other systems, as the springs will always revert to their default position, applying the brakes. This design is fundamental in heavy vehicle braking systems where safety is paramount.