Abstract: A fluid pump includes a pump body enclosing a first cavity and a second cavity, a first flexible member disposed within the first cavity, a second flexible member disposed within the second cavity, and a drive shaft extending between and attached to each of the first flexible member and the second flexible member. The drive shaft is configured to slide back and forth within the pump body. The pump also includes a first shift valve and a second shift valve disposed between the first flexible member and the second flexible member, operatively coupled to deliver a drive fluid to drive fluid chambers in alternating sequence. Some fluid pumps disclosed herein include a housing defining a modular-receiving cavity and a modular insert secured within the modular-receiving cavity by an interference fit. Methods of manufacturing and using fluid pumps are also disclosed.