The parking brake is a disk or drum, held in the applied position by spring pressure via a brake pot. When the vehicle’s engine is started, air pressure builds and this releases the ‘spring brake’. If there’s no air pressure, the brakes are on by default. This helps stop a disconnected trailer from running away.
IAM (Autodesk Inventor, filesize: 22.3 MB), STP (STEP, filesize: 22.3 MB)