Abstract: A balanced vane type hydraulic pump has a thrust plate and a pressure plate which cooperate with a cam ring, rotor and vanes to provide a plurality of pump chambers. During pump operation, fluid trapped under the vanes flows from under the vanes at the discharge port, as the vane recedes, to under the vanes at the inlet port to assist vane extension and improve pump priming. The undervane fluid passes through grooves in both plates with the groove in the pressure plate being in communication with the discharge flow of the pump. The grooves incorporate restrictions to the undervane fluid flow between the discharge and inlet positions of the vanes with the restriction in the thrust plate permitting more fluid flow than the restriction in the pressure plate to ensure that most of the fluid will pass through the rotor under the vanes in the inlet position to assist in vane extension.

Abstract: An engine driven vacuum pump combining a two-stage, low noise demountable pump assembly with a camshaft driven eccentric drive arrangement having internal oil distribution and pumping means for lubricating the drive and pump pushrod elements. A check valve in the cover of the pump assembly defines a second stage pumping chamber which increases pump efficiency while at the same time cyclically closing the pump outlet to reduce external noise attributable to gas discharge from the primary pumping chamber.