Abstract: A balanced rotary sliding vane pump has a flexible cheek plate slightly spaced from the rotor and vanes. A hydrostatic pressure pad exposed to outlet pressure urges the cheek plate to deflect toward the rotor and is fully effective at low speeds. A pressure force opposing this deflection at higher speeds is produced by restricting the outlet for fluid discharged by the inwardly moving vanes, thus augmenting the pressure field applied in the clearance space between the rotor and the cheek plate. This speed responsive change in the cheek plate deflection provides for a high volumetric efficiency at slow speeds and also reduces the possibility of cheek plate wear and seizure at higher speeds when volumetric efficiency is less important.