A hydraulic device for controlling braking for farm tractors, earthmovers and like machines, comprises a master cylinder and brake booster. A piston of the brake booster defines first and second chambers which, under rest conditions, are in communication and, during braking, are cut off with the first chamber being put in communication with a discharge chamber of the master cylinder. The piston of the master cylinder is driven by a drive piston shaped to define with a plunger of the piston of the brake booster a hollow space through which the first chamber communicates with either the second chamber or the discharge chamber, depending on device operating conditions. In the plunger, a seat for a gasket-valve is arranged to cooperate with the drive piston, during braking, to prevent fluid leakage between the second and the first chamber as a consequence of the sliding of the drive piston caused by braking.