How does the Spring-Loaded Parking Brake Cylinder (Federspeicherbremszylinder) function in a truck?

1. It uses a heavy mechanical coil spring to apply the parking brake; compressed air pressure (Circuit 3) is required to overcome the spring and release the brake
2. It requires continuous high hydraulic oil pressure to apply the brake
3. It applies braking force only when the driver steps on the accelerator
4. It works purely through magnetic attraction