Pressure balancing system for a pair of tandem master cylinders, each of said tandem master cylinders comprising within a hollow body: - a primary plunger, - a secondary plunger, - a primary chamber, between said primary plunger and said secondary plunger, - a secondary chamber, between said secondary plunger and a bottom of the hollow body, and - a secondary preload sub-assembly, arranged within said secondary chamber and comprising a secondary preload pin and a secondary preload case, said pressure balancing system comprising within each of said tandem master cylinders - a primary balancing system comprising - a primary balancing duct provided in the hollow body of the tandem master cylinder, which primary balancing duct communicates a primary balancing channel located between the pair of tandem master cylinders with the inside of the hollow body at a region occupied by the secondary plunger when said secondary plunger is in an initial rest position, - a primary balancing o-ring arranged for forming a primary balancing valve between the primary balancing duct and the primary chamber, said primary balancing valve being closed when the secondary plunger is in said initial rest position, and characterised by being arranged for opening when the secondary plunger travels a predetermined portion of its stroke; and in that said pressure balancing system comprises, within each of said tandem master cylinders - a secondary balancing system comprising - a secondary balancing duct provided within the secondary preload pin and having a first end connected to a secondary balancing duct located between the pair of tandem master cylinders and a second end ending with at least one radial hole, - a secondary balancing o-ring, which is fixed to the secondary preload case and is arranged for forming, by cooperating in a tight manner with said at least one radial hole of the secondary balancing duct, a secondary balancing valve between the secondary balancing duct and the secondary chamber, said secondary balancing valve being closed when the secondary plunger is in said initial rest position and being arranged for opening when the secondary plunger travels a predetermined portion of its stroke such that the secondary balancing o-ring does not obstruct said at least one radial hole any longer. - a primary plunger, - a secondary plunger, - a primary chamber, between said primary plunger and said secondary plunger, - a secondary chamber, between said secondary plunger and a bottom of the hollow body, and - a secondary preload sub-assembly, arranged within said secondary chamber and comprising a secondary preload pin and a secondary preload case, - a primary balancing system comprising - a primary balancing duct provided in the hollow body of the tandem master cylinder, which primary balancing duct communicates a primary balancing channel located between the pair of tandem master cylinders with the inside of the hollow body at a region occupied by the secondary plunger when said secondary plunger is in an initial rest position, - a primary balancing o-ring arranged for forming a primary balancing valve between the primary balancing duct and the primary chamber, said primary balancing valve being closed when the secondary plunger is in said initial rest position, and - a primary balancing duct provided in the hollow body of the tandem master cylinder, which primary balancing duct communicates a primary balancing channel located between the pair of tandem master cylinders with the inside of the hollow body at a region occupied by the secondary plunger when said secondary plunger is in an initial rest position, - a primary balancing o-ring arranged for forming a primary balancing valve between the primary balancing duct and the primary chamber, said primary balancing valve being closed when the secondary plunger is in said initial rest position, and - a secondary balancing system comprising - a secondary balancing duct provided within the secondary preload pin and having a first end connected to a secondary balancing duct located between the pair of tandem master cylinders and a second end ending with at least one radial hole, - a secondary balancing o-ring, which is fixed to the secondary preload case and is arranged for forming, by cooperating in a tight manner with said at least one radial hole of the secondary balancing duct, a secondary balancing valve between the secondary balancing duct and the secondary chamber, said secondary balancing valve being closed when the secondary plunger is in said initial rest position and being arranged for opening when the secondary plunger travels a predetermined portion of its stroke such that the secondary balancing o-ring does not obstruct said at least one radial hole any longer. - a secondary balancing duct provided within the secondary preload pin and having a first end connected to a secondary balancing duct located between the pair of tandem master cylinders and a second end ending with at least one radial hole, - a secondary balancing o-ring, which is fixed to the secondary preload case and is arranged for forming, by cooperating in a tight manner with said at least one radial hole of the secondary balancing duct, a secondary balancing valve between the secondary balancing duct and the secondary chamber, said secondary balancing valve being closed when the secondary plunger is in said initial rest position and being arranged for opening when the secondary plunger travels a predetermined portion of its stroke such that the secondary balancing o-ring does not obstruct said at least one radial hole any longer.