the basis of a travel sensor 25 which is used in the tandem master cylinder 16 and has a transformer at this point it is to be noted that the explanation below is purely by way of example and that the invention can be applied to any transformers reference is made to fig2 which shows the tandem master cylinder 16 with a travel sensor 25 the tandem master cylinder 16 also has a pressure piston 26 of the service brake request 22 is movably arranged in a housing 28 wherein the pressure piston 26 is connected to the brake pedal 20 from fig1 in a way which is not illustrated in more detail the pressure piston 26 itself is divided into a primary piston 30 and a secondary piston 32 wherein the primary piston 30 closes off an inlet of the housing 28 and the secondary piston 32 divides the interior of the housing 28 into a primary chamber 34 and a secondary chamber 36 at the primary piston 30 a secondary collar 38 which separates the interior of the housing 28 from the surrounding air is arranged in the region of the inlet of the housing 28 looking into the interior of the housing 28 the secondary collar 38 is followed by a primary collar 40 which seals the gap between the primary piston 30 and a wall of the housing 28 a pressure collar 42 on the secondary piston 32 separates the pressure of the primary chamber 34 from the pressure of the secondary chamber 36 in addition a further primary collar 40 on the secondary piston 32 seals a gap between the secondary piston 32 and the wall of the housing 28 the primary.