According to their functions, there should be multiple check valves installed on the machine. There is a check valve between the main exhaust pipe and the oil separator, and another one on the pressure maintenance valve. The former prevents oil from flowing back into the air end after the machine shuts down, while the check valve on the pressure maintenance valve prevents external pressure from flowing back into the fine oil separator or the air end.
Furthermore, there is also a check valve installed on the exhaust outlet pipe after passing through the cooler. Its purpose is to prevent compressed air from the main air supply line from flowing back into the air compressor unit when the compressor is not in operation.