In a hydraulic pump assembly, an annular sleeve member is radially interposed between an outer peripheral surface portion of an input shaft and an inner peripheral surface portion of a cylinder block or barrel within which are provided a plurality of piston cylinders for accommodating a plurality of pumping pistons. The outer peripheral surface portion of the input shaft includes an outer peripheral splined section, and the inner peripheral surface portion of the cylinder block or barrel includes an inner peripheral splined section. The annular sleeve member is provided with the inner peripheral splined portion for splined engagement with the outer peripheral splined portion of the input shaft and with the outer peripheral splined portion for engagement with the inner peripheral splined portion of the cylinder block or barrel. In this manner, a dual splined interconnection system is defined between the input shaft, the annular sleeve member and the cylinder block or barrel.