Abstract: A solvent delivery pump assembly for use in high performance liquid chromatography systems, which accurately delivers a solvent (eluent) drawn from a reservoir, and having removed gases, dissolved in the eluent, and then delivering the eluent as desired for analysis. The apparatus comprises two pump units, each with two plungers of different diameters on a single shaft; one of the plungers in one of the pump unit is of a large diameter size for driving a first pump head in large volume, and the other plunger in the other pump unit is of a smaller diameter for driving a second pump head in small volume.