An electromechanical brake power assist unit of a vehicle braking system has a master brake cylinder with a piston rod, displaceable by a brake pedal and, in the process, building up brake pressure. The piston rod is also displaceable by an electric motor. The piston rod is supported on a pedal lever, which is formed by the brake pedal itself or an intermediate element pivotable about an axis of rotation by the brake pedal. The piston rod is displaced by a pivotal movement of the pedal lever. An axis of rotation of the pedal lever is displaceable by the electric motor in a moving direction of the piston rod. The pedal ratio is selected such that the driver can slightly displace the piston rod by forces customary in the case of a conventional effective brake power assistance also without an activation of the electric motor, and the pedal path which is large, because of such a pedal ratio without an activation of the electric motor, will then be reduced to a normal extent by the electric-motor-driven displacement of the pedal lever axis rotation.