Braking system for motor vehicles and method for operating the same

A brake system for motor vehicles having multiple wheel brakes, which brake system can be actuated both by a vehicle driver in a brake by wire operating mode and also independently of the vehicle driver in the brake by wire operating mode, which is normally operated in the brake by wire operating mode, and can be operated in at least one fall-back operating mode in which only actuation by the vehicle driver is possible, comprising,

• a brake pedal for actuating a pedal decoupling unit having a housing, with two pistons which are arranged in series and which, in the housing, delimit two pressure chambers, the pistons are subjected to an actuating force when the brake pedal is actuated by the driver, and the pistons are positioned in an initial position by restoring springs when the brake pedal is not actuated,
• a pressure medium reservoir which is assigned to the pressure chambers and which is at atmospheric pressure,
• a travel measuring device which measures the actuating travel of the brake pedal or of one of the pistons which is connected to the brake pedal,
• a travel simulator with a simulator enable valve which, in the brake by wire operating mode, enables a haptic brake pedal feel to the vehicle driver, the travel simulator being hydraulically connected to one of the pressure chambers, and the action of travel simulator can be deactivated in the fall-back operating mode,
• an electrohydraulic pressure generating device which outputs a brake system pressure,
• a pressure modulation unit which has an inlet valve and an outlet valve for each corresponding wheel brake of the wheel brakes of the vehicle for setting wheel-specific brake pressures derived from the brake system pressure, wherein the inlet valves transmit the brake system pressure when in a non-actuated state,
• wherein the wheel brakes are arranged in at least a first circuit and a second circuit, the first circuit having two associated inlet valves being connected to each other via a first system pressure line, and the second circuit having two associated inlet valves being connected to each other via a second system pressure line;
• cut-off valves for isolating the pressure chambers from the pressure modulation unit, and
• an electronic control and regulating unit,
• a first electrically actuatable activation valve for the first brake circuit, the first electrically actuatable activation valve being interposed in terms of circuit layout between the pressure generating device and the pressure modulation unit, the first activation valve, when in an actuated state, providing a hydraulic connection between the pressure generating device and the first system pressure line;
• a second electrically actuatable activation valve for the second brake circuit, the second electrically actuatable activation valve being interposed in terms of circuit layout between the pressure generating device and the pressure modulation unit, the second activation valve, when in an actuated state, providing a hydraulic connection between the pressure generating device and the second system pressure line;
• wherein the first electrically actuatable valve is the only valve disposed between the pressure generating device and the first system pressure line and the second electrically actuatable valve is the only valve disposed between the pressure generating device and the second system pressure line.