Final answer:
The parking and emergency brakes are usually held back by spring pressure.
Step-by-step explanation:
In normal operations, the parking and emergency brakes are usually held back by spring pressure. When the parking brake lever or pedal is engaged, it applies pressure to the brake shoes or pads, which press against the drum or disc rotors to prevent the vehicle from rolling. This pressure is provided by a set of springs that hold the brakes in place until released.