A valve operating mechanism for use in a fluid dispenser is described, wherein the fluid dispenser includes a reservoir for a fluid to be delivered, a fluid outlet for the reservoir, a piston configured to move through the reservoir, and an actuator for the piston, the valve operating mechanism including a valve, wherein the valve has an upstream side and a downstream side, and in at least one point of operation of the valve operating mechanism, the valve is inoperable as a consequence of the pressure difference between the upstream side of the valve and the downstream side of the valve.