Abstract: A positive displacement pump comprising a pump housing, a pump rotor disposed in the housing together with pressure plates on either side of the rotor arranged in a compact assembly, an end plate in the assembly held fast within the pump housing in concentric disposition with respect to the pump rotor, the housing and the innermost pressure plate defining a high pressure cavity within which is disposed a flat Belleville spring which preloads the plates and the rotor thereby pressure sealing the pump elements, a drive shaft sealed by a sealing element located in a lubrication oil cavity which communicates through an internal lube pressure passage with the inlet side of the pump, a fluid reservoir surrounding the pump and the valve assembly, the interior of the reservoir being semi-isolated from the low pressure lubrication oil cavity whereby the volume of metal required to form the housing and the axial dimensions of the pump assembly are reduced to minimum values.