the brake system includes a tandem master cylinder and a control valve for introducing pressure supplied from a power source . the main feature of the present invention is that the pressure from the power source is first introduced into annular chambers behind the sleeve seals of both the tandem master - cylinder pistons . this makes it possible for the pistons to travel only a small distance if power from the power source is available . thus , the pistons can be dimensioned in accordance with the force the driver is able to exert with his foot in the event of failure of the power source .