Abstract: The valve comprises three parts of generally cylindrical shape, of two or more diameters, fitted into one another, the first of these having a structure of two successive cylinders, being fixed between the cap of the container carrying the substance to be metered and the rim of the neck of the container itself, while the second part is intended to act as main plunger, sliding by its upper portion inside the first part and having a terminal end of smaller diameter guided in the portion of smaller diameter of the first part and having in its intermediate region a likewise intermediate diameter with the main filling and discharge vents for the product, and the third part is formed by a hollow rod rigidly connected to the handle/outlet nozzle and which likewise has a cylindrical structure with two successive different diameters.