Abstract: A brake force control dievice for two-circuit brake system includes a controlling piston disposed in a constant diameter internal chamber of a housing coaxial of a longitudinal axis, a locking piston disposed in the internal chamber coaxial of the axis and within the controlling piston and a third piston disposed in the internal chamber coaxial of the axis and between the controlling piston and the locking piston. The third piston slides on the locking piston and the controlling piston slides on at least a portion of the third piston. The controlling piston reduces the pressure of the rear axle brake circuit in relation to the pressure of the front axle brake circuit and the locking piston acts on a control valve in case of failure of the front axle brake circuit in such a way that the reducing effect for the rear axle brake pressure is neutralized.