According to the invention, the spring-effect deformation capacity of the valve (5) essentially relates to the plug (Sa) and to an intermediate valve portion (5c) between the plug (Sa) and a tubular gap (6). The valve (5) and the fixed end piece (3) are conformed so that the valve can be permanently tightened on the fixed end piece without forming a significant dead volume in said gap independently from the deformation of the valve, and so that the valve can be maintained on the fixed end piece (3) without any flexion under the action of the mobile end piece. The slot (9) of the valve includes a moulded proximal slot (9A) followed by a pierced distal slot (9B) for communication between the moulded slot and said gap when the slot is open. Application in medical fluid connectors.
