Abstract: A double-displacement pump is compact, has a modular fluid subassembly made up a substantially cylindrical pump body and a piston reciprocally driven within the pump body. An outlet fluid cavity is formed between the pump body and a surrounding housing structure, enabling the subassembly to be removed as a unit without the need to disconnect auxiliary systems, such as a pressure control system or a priming system. In a preferred embodiment, the subassembly is retained within the housing structure by an inlet fitting which includes a check valve. A second check valve is then provided in the piston itself, which has a stem threadingly engaged with a reciprocal drive element. A sleeve received over the stem serves to compress a fluid sealing structure against a flange of the piston when the stem is fully engaged with the drive element.