Brake control unit having a primary piston defining a primary chamber connected to a primary circuit and a secondary piston defining a secondary chamber connected to a secondary circuit, the control unit receiving the braking demand signal supplied via the push-rod actuated by the driver, said demand signal being detected by the displacement sensor for the movement of the push-rod in order to control the primary piston, which itself pushes against the secondary piston, thereby generating a pressure inside the primary chamber which then exerts a hydraulic thrust on the rear cross section of the secondary piston, said unit being characterized by - a plunger piston carried by the primary piston, being integral in translation with the latter and penetrating freely into the secondary piston, such that the primary chamber is defined between the primary piston, the secondary piston in their bore and the plunger piston, - the primary piston having a primary pressure seal (D1a), the secondary piston having a secondary pressure seal (D2b), and the plunger piston being equipped with a primary insulating seal (D1b) for its engagement in the secondary piston, such that - the primary piston has a front hydraulic cross section equal to the difference in its primary pressure cross section (S1a) and in its primary insulating cross section (S1b), and - the secondary piston has a rear hydraulic cross section equal to the difference between its secondary insulating cross section (S2a) and its primary insulating cross section (S1b), - and the front cross section (S2b) of the secondary piston is connected to the equivalent cross section (Se) connecting the inlet force (F IN) to the secondary pressure (Ps) by the equation S 1 b = A Se − B Se − C said equation indicating the primary insulating cross section (S1b) and the diameter of the primary insulating seal (D1b) as a function of the equivalent cross section (Se). - a plunger piston carried by the primary piston, being integral in translation with the latter and penetrating freely into the secondary piston, such that the primary chamber is defined between the primary piston, the secondary piston in their bore and the plunger piston, - the primary piston having a primary pressure seal (D1a), the secondary piston having a secondary pressure seal (D2b), and the plunger piston being equipped with a primary insulating seal (D1b) for its engagement in the secondary piston, such that - the primary piston has a front hydraulic cross section equal to the difference in its primary pressure cross section (S1a) and in its primary insulating cross section (S1b), and - the secondary piston has a rear hydraulic cross section equal to the difference between its secondary insulating cross section (S2a) and its primary insulating cross section (S1b), - and the front cross section (S2b) of the secondary piston is connected to the equivalent cross section (Se) connecting the inlet force (F IN) to the secondary pressure (Ps) by the equation S 1 b = A Se − B Se − C said equation indicating the primary insulating cross section (S1b) and the diameter of the primary insulating seal (D1b) as a function of the equivalent cross section (Se). - the primary piston has a front hydraulic cross section equal to the difference in its primary pressure cross section (S1a) and in its primary insulating cross section (S1b), and - the secondary piston has a rear hydraulic cross section equal to the difference between its secondary insulating cross section (S2a) and its primary insulating cross section (S1b), - and the front cross section (S2b) of the secondary piston is connected to the equivalent cross section (Se) connecting the inlet force (F IN) to the secondary pressure (Ps) by the equation S 1 b = A Se − B Se − C