A hydraulic braking system for a motor vehicle actuated by a braking pedal, comprising a master cylinder and a brake booster, wherein the brake booster includes: - a boost piston including a dual diameter plunger housing a hydraulic operating piston, said boost piston and said hydraulic operating piston shaping a slidable piston assembly (15, 20) configured for defining, in a body of the brake booster, a first and a second chamber (25, 28) arranged to be supplied with pressurised hydraulic fluid; - means associated with the piston assembly (15, 20) for setting up and cutting off, depending on the operation phases of the brake booster, a communication between the first chamber and the second chamber and a communication between the second chamber and an exhaust chamber; - means (41, 43) for transferring to a slidable piston of the master cylinder a braking force, applied to a brake pedal connected to the piston assembly (15, 20) and boosted in the brake booster, and for transferring to the piston assembly (15, 20) of the brake booster a reaction force exerted on the piston of the master cylinder, consequently to a sliding of the same piston caused by the braking force, by hydraulic fluid present in said master cylinder; wherein said means for transferring a braking force include - an elastomeric disc arranged between the piston assembly (15, 20) and a pushing member for the piston of the master cylinder and arranged to be deformed either by the piston assembly (15, 20) for transferring the braking force to the piston of the master cylinder or directly by the pushing member for transferring the reaction force to the piston assembly (15, 20), respectively; characterised by - a gasket positioned along a body of the master cylinder, and - a partition sealingly mounted in the body of the brake booster, said gasket and said partition, being placed between the exhaust chamber and a closed end of the body of the master cylinder, are arranged to define, with the end of the body of the master cylinder, a separation chamber between a first and a second mutually incompatible hydraulic fluid and mutually isolate spaces in which the first or the second fluid, respectively, are present; the separation chamber being provided with a discharge duct for discharging fluid leaked from either space; and in that - said elastomeric disc has first area and said hydraulic operating piston has a second area, said first area being larger than said second area and the ratio between said first area and said second area being arranged both to stabilise the pressure in the second chamber and obtain that the force applied to the master cylinder be a multiple of the force applied to the pedal depending on the ratio between said first area and said second area. - a boost piston including a dual diameter plunger housing a hydraulic operating piston, said boost piston and said hydraulic operating piston shaping a slidable piston assembly (15, 20) configured for defining, in a body of the brake booster, a first and a second chamber (25, 28) arranged to be supplied with pressurised hydraulic fluid; - means associated with the piston assembly (15, 20) for setting up and cutting off, depending on the operation phases of the brake booster, a communication between the first chamber and the second chamber and a communication between the second chamber and an exhaust chamber; - means (41, 43) for transferring to a slidable piston of the master cylinder a braking force, applied to a brake pedal connected to the piston assembly (15, 20) and boosted in the brake booster, and for transferring to the piston assembly (15, 20) of the brake booster a reaction force exerted on the piston of the master cylinder, consequently to a sliding of the same piston caused by the braking force, by hydraulic fluid present in said master cylinder; - an elastomeric disc arranged between the piston assembly (15, 20) and a pushing member for the piston of the master cylinder and arranged to be deformed either by the piston assembly (15, 20) for transferring the braking force to the piston of the master cylinder or directly by the pushing member for transferring the reaction force to the piston assembly (15, 20), respectively; - a gasket positioned along a body of the master cylinder, and - a partition sealingly mounted in the body of the brake booster, - said elastomeric disc has first area and said hydraulic operating piston has a second area, said first area being larger than said second area and the ratio between said first area and said second area being arranged both to stabilise the pressure in the second chamber and obtain that the force applied to the master cylinder be a multiple of the force applied to the pedal depending on the ratio between said first area and said second area.