A device for adjusting the free stroke of a piston of a hydraulic brakes for bicycles or motorcycles of the type comprising: a hydraulic actuator element, a pump assembly comprising a cylinder having an outlet and a piston for operating said hydraulic actuator element, said pump assembly comprising a fluid outlet and a piston and a communication line for a working fluid, the communication line being set between said pump assembly and said actuator element, the device comprising: a variable-volume chamber, designed to be set along said communication line between said pump assembly and said actuator element; an element configured to be mobile in response to entry of working fluid coming from the fluid outlet of said pump assembly so as to pass from a first position in which said variable-volume chamber is at least partially empty of fluid and a second position in which said variable-volume chamber has been filled with an amount of fluid, said device further comprising a hollow cylindrical sleeve configured to communicate with said fluid outlet and a cylindrical adjuster external to the hollow cylindrical sleeve characterized in that said variable-volume chamber is constituted by a gap defined between the outer surface of said hollow cylindrical sleeve and the inner surface of the cylindrical adjuster and wherein said hollow cylindrical sleeve has a first cavity configured to communicate with said hydraulic outlet and a second cavity communicating with the first cavity and suited for receiving a fast-coupling nozzle of said communication line. a hydraulic actuator element, a pump assembly comprising a cylinder having an outlet and a piston for operating said hydraulic actuator element, said pump assembly comprising a fluid outlet and a piston and a communication line for a working fluid, the communication line being set between said pump assembly and said actuator element, the device comprising: a variable-volume chamber, designed to be set along said communication line between said pump assembly and said actuator element; an element configured to be mobile in response to entry of working fluid coming from the fluid outlet of said pump assembly so as to pass from a first position in which said variable-volume chamber is at least partially empty of fluid and a second position in which said variable-volume chamber has been filled with an amount of fluid, said device further comprising a hollow cylindrical sleeve configured to communicate with said fluid outlet and a cylindrical adjuster external to the hollow cylindrical sleeve characterized in that said variable-volume chamber is constituted by a gap defined between the outer surface of said hollow cylindrical sleeve and the inner surface of the cylindrical adjuster and wherein said hollow cylindrical sleeve has a first cavity configured to communicate with said hydraulic outlet and a second cavity communicating with the first cavity and suited for receiving a fast-coupling nozzle of said communication line. a variable-volume chamber, designed to be set along said communication line between said pump assembly and said actuator element; an element configured to be mobile in response to entry of working fluid coming from the fluid outlet of said pump assembly so as to pass from a first position in which said variable-volume chamber is at least partially empty of fluid and a second position in which said variable-volume chamber has been filled with an amount of fluid, said device further comprising a hollow cylindrical sleeve configured to communicate with said fluid outlet and a cylindrical adjuster external to the hollow cylindrical sleeve characterized in that said variable-volume chamber is constituted by a gap defined between the outer surface of said hollow cylindrical sleeve and the inner surface of the cylindrical adjuster and wherein said hollow cylindrical sleeve has a first cavity configured to communicate with said hydraulic outlet and a second cavity communicating with the first cavity and suited for receiving a fast-coupling nozzle of said communication line.