Abstract: The present invention relates to methods and devices for providing a reduced risk of free flow from medical devices such as an infusion pump. A medicament containing reservoir is provided with a piston having a sealing portion formed by a material of a first stiffness. The piston further comprises a frictional enhancing member having one or more reservoir wall engaging portions and being at least partly made of a material having a stiffness greater than the first stiffness. By arranging the frictional enhancing member to exert a radially outwards directed force component against the cylindrical wall an increased friction is provided between the piston and the reservoir wall.