A positive displacement pump comprising a cam for driving a piston assembly. The cam having an odd number of lobes, a first face, a second opposing face, and a peripheral surface extending between the first face and the second face. Each of the first face and the second face having a shoulder formed thereon conforming to the contour of the peripheral surface. The piston assemblies having at least two connector members extending from a piston in a spaced apart, parallel relationship to one another. A compression stroke bearing extends between the connector members in rolling contact with the peripheral surface, a first retraction bearing extends inwardly from one of the connector members and is rollingly positioned in contact with the shoulder of the first face, and a second retraction bearing extends inwardly from another one of the connector members and is rollingly positioned in contact with the shoulder of the second face.