Protective circuit for parking brake system for commercial vehicles with device for trailer or semi-trailer mode

The vehicle has two service brake circuits with compressed air vessel for the tractor unit, a parking brake circuit, and a trailer/semi-trailer brake circuit with compressed air vessel (31) in the tractor unit. This compressed air vessel is connected via a back pressure valve (32) to a multi-circuit protection valve (13), compressed air can fill the third air vessel, and this supplies compressed air for the trailer/semi-trailer brake circuit only, so that this is separate from the air consumption for the tractor unit parking brake system only.