Brake system and method for operating such a brake system

1. A brake system for a motor vehicle with at least four hydraulically activated wheel brakes, comprising:

for each of the wheel brakes a first electrically activated wheel valve which is designed to be open when de-energized and a second electrically activated wheel valve which is designed to be closed when de-energized,

a first electrically activated pressure source, which is connected to each of the first electrically activated wheel valves via a first brake supply line, wherein arranged in the first brake supply line is an electrically activated circuit isolating valve by means of which two of the first electrically activated wheel valves can be hydraulically disconnected from the first pressure source,

a second electrically activated pressure source, and

a pressure medium reservoir vessel which is at atmospheric pressure,

wherein

the circuit isolating valve is designed to be open when de-energized, and the second electrically activated pressure source is connected to each of the second electrically activated wheel valves via a second brake supply line such that the second electrically activated pressure source is connected via each of the second electrically activated wheel valves to a respective one of the wheel brakes,

the brake system comprises a first electronic device by which the first pressure source is activated, and a second electronic device by which the second pressure source is activated, the second electronic device being electrically independent of the first electronic device,

the first and second electrically activated wheel valves are activated exclusively by the second electronic device, and

the circuit isolating valve is activated exclusively by the first electronic device.

2. The brake system as claimed in claim 1 , wherein there is no electrically activated valve arranged between the second pressure source and the second electrically activated wheel valves in the second brake supply line.

3. The brake system as claimed in claim 1 , wherein there is no valve arranged between the first pressure source and the first electrically activated wheel valves.

4. The brake system as claimed in claim 1 , wherein a wheel speed sensor is provided for each wheel brake, wherein the signals of the wheel speed sensors are fed to the second electronic device for evaluation, and the wheel speed sensors are supplied with electrical energy by the second electronic device.

5. The brake system as claimed in claim 1 , further comprising a vehicle movement dynamics sensor system, wherein the signals of the vehicle movement dynamics sensor system are fed to the first electronic device for evaluation, and the vehicle movement dynamics sensor system is supplied with electrical energy by the first electronic device.

6. The brake system as claimed in claim 1 , wherein at least the first pressure source is embodied as a cylinder-piston assembly with a pressure space which is bounded by a piston which can be moved, by an electric motor and a rotational-translational transmission, in an activation direction to build up pressure and in a direction opposite to the activation direction in order to reduce pressure.

7. The brake system as claimed in claim 6 , wherein in an unactivated state of the piston the pressure space is connected to the pressure medium reservoir vessel via at least one snifter hole, wherein this connection is interrupted when the piston is activated.

8. The brake system as claimed in claim 6 , wherein a resetting element, which positions the piston in the unactivated state when the electric motor is de-energized, is provided in the pressure space.

9. The brake system as claimed in claim 1 , wherein said brake system further comprises a simulation device which can be activated by a brake pedal, wherein no mechanical and/or hydraulic operative connection between the brake pedal and the wheel brakes is provided.

10. A method for operating a brake system as claimed in claim 1 , wherein in the event of a failure of the first pressure source or of the first electronic device, the first electrically activated wheel valves are closed by the second electronic device and the second electrically activated wheel valves are opened and the second pressure source is activated to build up a pressure.

11. The method as claimed in claim 10 , wherein in the event of a failure of the first pressure source or of the first electronic device a wheel-specific brake pressure closed-loop control process is carried out by the second electronic device, wherein a reduction in pressure is carried out at one of the wheel brakes by opening the corresponding first electrically activated wheel valve, wherein pressure medium flows away to the pressure medium reservoir vessel via the circuit isolating valve and the first pressure source.

12. The method for operating a brake system as claimed in claim 10 , wherein in the event of a failure of the second pressure source or the second electronic device the first pressure source is activated by the first electronic device to build up a brake pressure, and the circuit isolating valve is activated to set circuit-specific brake pressures.

13. The brake system as claimed in claim 1 , wherein there is no valve arranged between the second pressure source and the second electrically activated wheel valves in the second brake supply line.

14. The brake system as claimed in claim 2 , wherein there is no valve arranged between the first pressure source and the first electrically activated wheel valves.

15. The brake system as claimed in claim 7 , wherein a resetting element, which positions the piston in the unactivated state when the electric motor is de-energized, is provided in the pressure space.

16. The method for operating a brake system as claimed in claim 11 , wherein in the event of a failure of the second pressure source or the second electronic device the first pressure source is activated by the first electronic device to build up a brake pressure, and the circuit isolating valve is activated to set circuit-specific brake pressures.