A master brake cylinder for a braking system of a vehicle has: a first filling volume, which is fillable with liquid, and whose first dimension is variable by adjusting at least one first piston wall; a second filling volume, which is fillable with liquid, and whose dimension is variable by adjusting at least one second piston wall; and a valve unit, the first filling volume being connected via the valve unit to the second filling volume in such a way that when the valve unit is switched into a first valve state, a joint first pressure chamber pressure prevails in the first filling volume and in the second filling volume.