A closed level control system includes an air dryer (10) which is mounted in a pressurized air line (4). The compressor output (16) can be connected to the pressurized air supply vessel (12) via the pressurized air line (4). Air from the atmosphere can be transferred via an intake valve (30) into the pressurized air supply vessel (12) via the compressor and the air dryer (10). Furthermore, pressurized air can be discharged into the atmosphere from the pressurized air supply vessel (12) via the air dryer (10) and the intake valve (30). There is a flow in the opposite direction through the air dryer than when filling the pressurized air supply vessel (12) from the atmosphere. In this way, an excellent regeneration of the air dryer (10) is ensured.