Diesel locomotives were introduced at a time when vacuum brakes were still widespread. Ejectors are not practical and so mechanical pumps or 'exhausters' are used instead. These are a small rotary vane pump, similar to some forms of vacuum pump. The body is a cylindrical metal casting with a cylindrical rotor within it, but the two axes are offset. The rotor contains a number of sliding vanes, typically six. As the rotor is rotated, the vanes are held against the walls of the cylindrical body. Entry and exit ports at the top and bottom of the cylinder, where the rotor is furthest from and nearest to the wall, provide a vacuum pumping effect. The vanes are held against the cylinder by an internal cam ring or by springs. They are lubricated by an oil feed into the exhauster. As the exhauster is oil-lubricated, the exhaust air is full of oil droplets and so passes through an oil separator before being exhausted to the atmosphere. A simple check valve on the inlet prevents backflow leakage, if the exhauster stops.