A positive displacement pump having a centrally disposed rotor within a housing, the rotor having inwardly and outwardly moving vanes for movement of fluid through the pump. Races in end walls of the housing partly or entirely control movement of the vanes. A dam is movably positioned at the housing's fluid discharge port, the dam having upper and lower ends, the lower end of the dam extending into the housing and being positioned so as to be close to or in sliding contact with the surface of the rotor. This pump construction is extremely versatile in its construction and applications.