The invention concerns a master cylinder assembly (10, 110) for balancing the braking of a couple of wheels (LBW, RBW) of a vehicle. Each master cylinder (12) has a hollow body (14), a piston (15) and a chamber (16) suitable for containing a fluid having a primary control pressure depending on a driving force applied on the piston (15) and intended to be transmitted to a utilising device (LBB, RBB). The assembly further includes a balancing duct (58) into which the chambers (16) come out, and a plurality of normally closed balancing valves (60). Each balancing valve (60) is located between the balancing duct (58) and a respective chamber (16) and has a shutter (62) which is mechanically unconstrained to the primary piston (15) defining the primary chamber (16) to which said primary balancing valve (60) is associated, and which is operable for opening in a manner fluid-controlled by the primary control pressure taken by the fluid contained in the primary chamber (16) to which said balancing valve (60) is associated.