Brake system for motor vehicles

1. A brake system for motor vehicles, which brake system can be actuated in a “brake-by-wire” operating mode both by the vehicle driver and independently of the vehicle driver, and can be operated in at least one fallback operating mode, the system comprising:

a master brake cylinder which can be actuated by way of a brake pedal and which has a housing and two pistons which, in the housing, delimit two pressure chambers which are assigned to respective brake circuits with circuits, each of the brake circuits having a respective plurality of wheel brakes,

a pressure medium reservoir,

an electrically actuable inlet valve for each wheel brake, for the setting of wheel-specific brake pressures,

a first electrically controllable pressure provision device for the pressurization of at least one of the plurality of wheel brakes of each of the brake circuits in the “brake-by-wire” operating mode,

a simulation device which, in the “brake-by-wire” operating mode, imparts an acceptable brake pedal feel to the vehicle driver, and

a second electrically controllable pressure provision device having at least one suction port and at least one pressure port, the suction port of which is connected, without a valve being connected in between, to the pressure medium reservoir,

wherein the at least one pressure port of the second pressure provision device is connected to at least one of the plurality of wheel brakes of each of the brake circuits without a valve being connected in between.

2. The brake system as claimed in claim 1 , wherein the second pressure provision device comprises at least two hydraulic circuits, wherein the at least one pressure port is provided for each of the at least two hydraulic circuits, and each pressure port is connected to the at least one of the plurality of wheel brakes of each of the brake circuits without a valve being connected in between.

3. The brake system as claimed in claim 2 , wherein the second pressure provision device comprises at least two hydraulic pumps which are associated with respective ones of the at least two hydraulic circuits and are driven jointly by one electric motor and which have in each case one suction port and one pressure port, wherein each of the pressure ports is connected to the at least one of the plurality of wheel brakes of each of the brake circuits without a valve being connected in between.

4. The brake system as claimed in claim 3 , wherein each of the pressure ports of the first and of the second pump is connected to precisely one of the plurality of wheel brakes of each of the brake circuits.

5. The brake system as claimed in claim 3 , wherein for the first and the second pump, in each case one electrically actuable valve, which is closed when deenergized and/or is analogized or actuated in analog fashion, is provided, which valve is connected in parallel with the pump.

6. The brake system as claimed in claim 1 , wherein the one or more pressure port(s) is/are connected to a connecting line between the wheel brake and the outlet-side port of the associated inlet valve.

7. The brake system as claimed in claim 1 , wherein a pressure detection device is provided which detects a pressure port-side pressure of the second pressure provision device.

8. The brake system as claimed in claim 1 , further comprising a first electronic control and regulation unit for the actuation of the first pressure provision device and a second electronic control and regulation unit for the actuation of the second pressure provision device, and of the valves, and for the evaluation of the output signals of the pressure detection device.

9. The brake system as claimed in claim 8 , wherein the second pressure provision device is arranged in an independent assembly, wherein the assembly additionally comprises at least one of the pressure detection device, the second electronic control and regulation unit, and the valves.