A metering mechanism comprising a metering body mounted to move in translation in a jacket itself mounted in an adjusting sleeve, said adjusting sleeve being adapted to cooperate by way of a screwing movement with said sleeve, screwing/unscrewing driving the metering body in translation, said body being provided at one of its ends with a shut-off check valve and accepting at the other of its ends a plunger piston the alternating movement in translation of which enables suction at the level of the end of the body provided with the check valve and then expulsion into a volume surrounding the other end of the body via a passage formed in at least one sealing device (J1, J93, J91), characterized in that: - the metering body includes a central first cylinder and a peripheral second cylinder that are concentric and fastened to one another, - the plunger piston is fastened to a first sealing device and a second sealing device respectively mounted to bear against the first and second cylinders, so that a reduced pressure zone is created inside the first and second cylinders, respectively, when the first and second sealing devices (J1; J93) are moved away from the end of the body provided with the check valve, - the reduced pressure zone of the central first cylinder is adapted to be caused to communicate with the reduced pressure zone of the peripheral second cylinder and the reduced pressure zone of the peripheral second cylinder being adapted to be caused to communicate with a volume surrounding the end of the body opposite that provided with the check valve, - the metering mechanism comprises a selector mounted on the central first cylinder and mobile relative thereto over a stroke delimited by a first position in which the communication between the reduced pressure zones of the first and second cylinders is cut off while the communication between the reduced pressure zone of the second cylinder and the volume surrounding the end of the body opposite that provided with the check valve is open and a second position in which the communication between the reduced pressure zones of the first and second cylinders is open while the communication between the reduced pressure zone of the second cylinder and the volume surrounding the end of the body opposite that provided with the valve is cut off. - the metering body includes a central first cylinder and a peripheral second cylinder that are concentric and fastened to one another, - the plunger piston is fastened to a first sealing device and a second sealing device respectively mounted to bear against the first and second cylinders, so that a reduced pressure zone is created inside the first and second cylinders, respectively, when the first and second sealing devices (J1; J93) are moved away from the end of the body provided with the check valve, - the reduced pressure zone of the central first cylinder is adapted to be caused to communicate with the reduced pressure zone of the peripheral second cylinder and the reduced pressure zone of the peripheral second cylinder being adapted to be caused to communicate with a volume surrounding the end of the body opposite that provided with the check valve, - the metering mechanism comprises a selector mounted on the central first cylinder and mobile relative thereto over a stroke delimited by a first position in which the communication between the reduced pressure zones of the first and second cylinders is cut off while the communication between the reduced pressure zone of the second cylinder and the volume surrounding the end of the body opposite that provided with the check valve is open and a second position in which the communication between the reduced pressure zones of the first and second cylinders is open while the communication between the reduced pressure zone of the second cylinder and the volume surrounding the end of the body opposite that provided with the valve is cut off.