A needle-free medical connector includes a housing with a first port and a second port. The connector also includes a piston element defining a fluid passageway between the first and second ports. The piston element is movable between flow and non-flow positions. The piston element has an expandable section having a variable inner width and volume that forms a part of the flow path through the connector. As the piston is compressed to the flow position, the expandable section is expanded in width by movement over a flow post thereby maintaining or increasing the volume of the fluid passageway through the connector. The expandable section has a configuration permitting the continuous flow of fluid through its entirety.