Disclosed is a level control system in which the operational readiness of the compressor (2) is tested by connecting the pressure sensor (10) to the output (6) of the compressor (2) via a compressed-air sensor line (8). All other compressed-air lines that are connected to the output (6) of the compressor (2) are locked by shutting directional control valves (201 to 20d and 30). The compressor (2) is then switched on and it is tested how rapidly the pressure increases on the pressure sensor (10). If the increase per interval exceeds a defined threshold value, the compressor is operationally ready.