Abstract: A modular pump features a gas-control-and-drive-unit (GCDU), a cartridge-style-pumping-module (CSPU) and a retainer clip (RC). The GCDU includes a GCDU-housing to receive/provide gas to a gas-passageway causing a GCDU-force in one direction; an actuator-rod-and-slide-valve-arrangement (AR/SVA) to respond to a CSPM-force and exhaust the gas from the GCDU-housing causing a GCDU-exhaustion-force in an opposite direction; and a GCDU-coupling-member. The CSPM includes a CSPM-housing to receive a fluid to be pumped, and a piston-and-dual-diaphragm-arrangement (PDDA) to respond to the GCDU-force, provide the CSPM-force and pump the fluid from the CSPM, and to respond to the GCDU-exhaustion-force and pump the fluid from the CSPM. The CSPM-housing includes a CSPM coupling member to detachably couple to the GCDU-coupling-member. The RC detachably couples a piston of the PDDA and an actuator rod of the AR/SVA, so the CSPM attaches/detaches from the GCDU using a three-step process for field replacement using the RC.