A cock for filling tanks, which comprises: a hollow head, inside which a valve device linked to a float by means of an intermediate device is housed, also comprising a vertical tube connected to the hollow head, where the vertical tube is housed inside a central passage in the float, which moves upwards and downwards according to a rising and falling water level in the tank; wherein the float is connected to a bush coaxially coupled around the vertical tube, along the length of which the bush moves freely and axially; the cock comprising furthermore a connecting rod; characterized in that the bush incorporates parallel runners, where the bush is moved and guided vertically in grooves arranged along the vertical tube; the parallel runners being linked to a lever, which is coupled to the hollow head via an articulated connection; wherein one end of the connecting rod is connected via a front articulated connection to the lever, whilst an opposite end of the connecting rod is connected to a push piston via a rear articulated connection, the push piston being linked to the valve device; and wherein the lever tilting in one direction axially moves the push piston against the valve device towards a closed position; the lever comprises a front fork whose end sections have opposite stubs which are fitted into and guided in corresponding slots of the parallel runners located in the upper portion of the bush, and wherein the lever comprises a double rear extension, which is coupled at its free end to one portion of the hollow head, via the articulated connection, whilst a connecting rod is also coupled to said double rear extension by the front articulated connection thereof. a hollow head, inside which a valve device linked to a float by means of an intermediate device is housed, also comprising a vertical tube connected to the hollow head, where the vertical tube is housed inside a central passage in the float, which moves upwards and downwards according to a rising and falling water level in the tank; wherein the float is connected to a bush coaxially coupled around the vertical tube, along the length of which the bush moves freely and axially; the cock comprising furthermore a connecting rod; characterized in that the bush incorporates parallel runners, where the bush is moved and guided vertically in grooves arranged along the vertical tube; the parallel runners being linked to a lever, which is coupled to the hollow head via an articulated connection; wherein one end of the connecting rod is connected via a front articulated connection to the lever, whilst an opposite end of the connecting rod is connected to a push piston via a rear articulated connection, the push piston being linked to the valve device; and wherein the lever tilting in one direction axially moves the push piston against the valve device towards a closed position; the lever comprises a front fork whose end sections have opposite stubs which are fitted into and guided in corresponding slots of the parallel runners located in the upper portion of the bush, and wherein the lever comprises a double rear extension, which is coupled at its free end to one portion of the hollow head, via the articulated connection, whilst a connecting rod is also coupled to said double rear extension by the front articulated connection thereof.