Lip seals retain the oil inside the air end while keeping air, dust and dirt from entering along the rotating shaft when the machine is off load. When the inlet valve closes and the shaft continues to rotate, a vacuum may be pulled on the inlet end. Compressors with these lip seals rely on discharge valves, oil stop valves and a fast acting sump blow down valve to prevent the system pressure from slipping back up through stopped rotors and pressurizing the drive side.