Method for operating a brake system and a brake system

A method for operating a brake system for motor vehicles, which can be controlled in a “brake-by-wire” operating mode both by the vehicle driver and also independently of the vehicle driver, is preferably operated in the “brake-by-wire” operating mode, and can be operated in at least one fall-back operating mode, with

• hydraulic wheel brakes
• a brake master cylinder having at least one pressure space, with the pressure in which at least the wheel brakes of a front axle of the motor vehicle can be supplied in the fall-back operating mode,
• a pressure medium reservoir,
• a brake pedal for actuating the brake master cylinder,
• brake circuits, which have brake master cylinder and wheel brake pressure spaces, hydraulic connections and electrohydraulic control elements,
• an electrically actuable isolating valve for each brake circuit for dividing the brake circuit into a brake circuit section which can be subjected to pressure by the vehicle driver and a brake circuit section which can be subjected to pressure “by wire”,
• an electrically actuable inlet valve and an electrically actuable outlet valve for each wheel brake for setting wheel-specific brake pressures, an outlet side of each outlet valve being connected without an intervening valve to an inlet side of a respective one of the inlet valves,
• an electrically controllable pressure and volume setting device,
• an electrically actuable connection valve, which is closed when deenergized, for each brake circuit for hydraulic connection of the electrically controllable pressure and volume setting device to the brake circuit section which can be subjected to pressure “by wire”,
• a simulation device, which can be connected hydraulically to at least one pressure space of the brake master cylinder and can be activated by means of an electrically or mechanically actuable simulator enable valve, having a preset characteristic, and which gives the vehicle driver a pleasant brake pedal feel in the “brake-by-wire” operating mode, and
• an electronic open- and closed-loop control unit, for controlling the pressure and volume setting device, the isolating valves, the connection valves, the simulator enable valve and the inlet and outlet valves,
• wherein, to set wheel-specific brake pressures, the inlet and outlet valves are controlled in an analog manner by being supplied with different electric currents in order to allocate a pressure medium volume flow made available by the pressure and volume setting device to the wheel brakes having a requirement for a change in pressure.