Brake system for motor vehicles and method for operating a brake system

A brake system for motor vehicles, which can be activated in a brake-by-wire operating mode both by a vehicle driver and independently of the vehicle driver, which is normally operated in the brake-by-wire operating mode, and can be operated in at least one fallback operating mode, the brake system comprising:

• a brake master cylinder having a housing and two pistons, which delimit two pressure spaces in the housing, the pressure spaces being assigned to first and second brake circuits with corresponding pairs of wheel brakes,
• a pressure medium reservoir having at least two chambers assigned to the first and the second brake circuits,
• a brake pedal for actuating the brake master cylinder, the pedal being coupled to a piston of the brake master cylinder,
• an electrically operable normally open isolation valve for each of the brake circuits for separating the brake circuit into a brake circuit section which can be pressurized by the vehicle driver and a brake circuit section which can be pressurized by an electronic control,
• an electrically operable normally open inlet valve and an electrically operable normally closed outlet valve for each wheel brake for setting wheel-specific brake pressures,
• a first electrically controllable pressure supply device, which is formed by a cylinder-piston arrangement, the piston of which can be actuated by an electromechanical actuator,
• an electrically operable normally closed sequence valve for each of the brake circuits for hydraulically connecting the first electrically controllable pressure supply device to the brake circuit section which can be pressurized by the electronic control,
• a simulation device, which can be connected hydraulically to at least one pressure space of the brake master cylinder by means of an electronically or mechanically operable simulator enabling valve and which, in the brake-by-wire operating mode, conveys a haptic brake pedal sensation to the vehicle driver, and
• a first electronic control unit for activating the first pressure supply device, the isolation valve, the sequence valve, and the simulator enabling valve,
• wherein a second electrically controllable pressure supply device having at least one intake port and one pressure port is provided, the at least one intake port of the second electrically controllable pressure supply device is connected directly without the interposition of a valve to the pressure medium reservoir or to a low-pressure accumulator and the pressure port of the second electrically controllable pressure supply device is connected to the brake circuit section of a brake circuit which can be pressurized by the electronic control.