The present disclosure is directed towards dosing unit for use in an ambulatory infusion system, the dosing unit including: a dosing cylinder and a piston arranged inside the dosing cylinder and in a sliding displaceable manner along a displacement axis. The piston is convertible form a storing configuration into an operational configuration, wherein a circumferential sealing member of the piston is mechanically relieved in the storing configuration and is in sealing and sliding engagement with a circumferential inner surface of the dosing cylinder in the operational configuration. The dosing unit includes a configuration switch member in operative mechanical coupling with the piston, the configuration switch member being movable relative to the piston from a storing position into an operational position, thereby switching the piston configuration from the storing configuration to the operational configuration.
