Abstract: A device for metered delivery of a liquid or viscous substance comprising a casing, a reciprocating differential piston able to slide in said casing, a metering chamber and at least one outlet for media delivery, the volume of the metering chamber varying during the piston stroke and comprising an inlet communicating with the outside of the casing via a first non-return valve, said piston comprising a central bore connecting said metering chamber by way of a central bore inlet and said at least one outlet via a passage, said central bore inlet being provided with a second non-return valve, said first non-return valve and second non-return valve being arranged face to face, piston stroke length being able to adjust the volume of the metering chamber.