A hydraulic device for controlling braking, in particular for operating machines such as farm tractors, earthmovers and the like, the device comprising at least one master cylinder and at least one brake booster formed in a common body, wherein: - a piston of the master cylinder defines a pressurised chamber of the master cylinder, connected to vehicle brakes; - a piston of the brake booster defines a first and a second pressurised chamber (39, 40) of the brake booster, associated with communication means arranged, under rest conditions, to establish a communication between the first and the second pressurised chamber (39, 40) of the brake booster and, during braking, to cut off such a communication and to establish a communication between the first chamber of the brake booster and a discharge chamber of the master cylinder; - the piston of the master cylinder is driven by a drive piston, which is mounted so as to tightly slide within a plunger of the piston of the brake booster and is operated as a consequence of the operation of a brake pedal; and - the drive piston is so shaped as to define, together with the plunger of the piston of the brake booster, a hollow space through which the first chamber of the brake booster is put in communication with either the second chamber of the brake booster or the discharge chamber of the master cylinder, depending on the operating conditions of the device; the device being characterised in that in said plunger of the piston of the brake booster, between a passageway putting the second chamber of the brake booster in communication with the hollow space and a passageway putting the hollow space in communication with the first chamber of the brake booster, there is provided a seat for a gasket - valve arranged to cooperate with the drive piston, during braking, in order to prevent fluid leakage between the second and the first chamber (40, 39) of the brake booster through the hollow space when the passageway putting the second chamber of the brake booster in communication with the hollow space is closed as a consequence of the sliding of the drive piston caused by braking. - a piston of the master cylinder defines a pressurised chamber of the master cylinder, connected to vehicle brakes; - a piston of the brake booster defines a first and a second pressurised chamber (39, 40) of the brake booster, associated with communication means arranged, under rest conditions, to establish a communication between the first and the second pressurised chamber (39, 40) of the brake booster and, during braking, to cut off such a communication and to establish a communication between the first chamber of the brake booster and a discharge chamber of the master cylinder; - the piston of the master cylinder is driven by a drive piston, which is mounted so as to tightly slide within a plunger of the piston of the brake booster and is operated as a consequence of the operation of a brake pedal; and - the drive piston is so shaped as to define, together with the plunger of the piston of the brake booster, a hollow space through which the first chamber of the brake booster is put in communication with either the second chamber of the brake booster or the discharge chamber of the master cylinder, depending on the operating conditions of the device;