Abstract: A pump has a pressure chamber and a control ring which is mounted to pivot about a pivoting axis and which undergoes a pivoting motion. The control ring can be pivoted about a deflection angle during its pivoting movement. An aperture is provided in the pressure chamber of the pump. The aperture opens or closes to varying degree based on the pivoting motion of the control ring. In a hydrodynamic retarder with such a pump, a lubricating and cooling medium is supplied at least to bearings of rotating components of the retarder. Differentiated lubrication and cooling, at least of the bearings, can be brought about during braking operation and during non-braking operation of the retarder.