A drive system for a pump includes a housing and a fluid displacer and a reciprocator configured to mechanically displace the fluid displacer through respective suction strokes. The housing and fluid displacer define an internal pressure chamber configured to be filled with a working fluid having a charge pressure. The internal pressure chamber is configured such that the working fluid exerts the charge pressure on the fluid displacer during both the suction stroke and a pressure stroke of the fluid displacer.