Method for operating a brake system for motor vehicles, and brake system

1. A method for operating a brake system for motor vehicles, the brake system comprising a master brake cylinder actuatable by a brake pedal, an electrically controllable pressure provision device, a pressure medium reservoir tank, from which the master brake cylinder and the pressure provision device are supplied with pressure medium, and at least two hydraulically actuatable wheel brakes and at least one electrically actuatable inlet valve per wheel brake, the method comprising:

monitoring the pressure medium reservoir tank by a device for determining a level of the pressure medium, wherein the at least two hydraulically actuatable wheel brakes are divided into at least one first wheel brake group and one second wheel brake group;

closing inlet valves of the first wheel brake group among the at least one electrically actuatable inlet valve when the determined level falls below a first limit value; and

opening the inlet valves of the first wheel brake group and closing inlet valves of the second wheel brake group of the at least one electrically actuatable inlet valve when the level falls below a second limit value, which is lower than the first limit value.

2. The method as claimed in claim 1 , wherein the at least two hydraulically actuatable wheel brakes comprises at least four wheel brakes, and

wherein the first wheel brake group comprises a first wheel brake and a second wheel brake of the at least four wheel brakes.

3. The method as claimed in claim 2 , wherein the second wheel brake group comprises a third wheel brake and a fourth wheel brake of the at least four wheel brakes.

4. The method as claimed in claim 2 , wherein the first wheel brake and the second wheel brake are arranged on opposite sides of the vehicle.

5. The method as claimed in claim 4 , further comprising performing a diagnostic routine for determining a leak in the pressure provision device when the level falls below a third limit value, which is lower than the second limit value.

6. The method as claimed in claim 5 , wherein the master brake cylinder is separably connected via an isolating valve to a brake supply line to which the at least four wheel brakes are connected and which is separably connected via an activation valve to the pressure provision device, and

wherein the inlet valves of the at least four wheel brakes, the isolating valve and the activation valve are closed during the diagnostic routine to pressurize the brake system.

7. The method as claimed in claim 5 , further comprising operating the brake system in a first fallback operating mode if the diagnostic routine shows that there is a leak in the pressure provision device, and

wherein, in the first fallback operating mode, the pressure provision device is separated from the at least four wheel brakes.

8. The method as claimed in claim 7 , wherein the pressure provision device is separated from the at least four wheel brakes by switching an activation valve, and the at least four wheel brakes are actuated by pressure medium from the master brake cylinder.

9. The method as claimed in claim 5 , further comprising operating the brake system in a second fallback operating mode if the diagnostic routine shows that there is no leak in the pressure provision device, and

wherein, in the second fallback operating mode, the at least four wheel brakes are actuated by pressure medium from the pressure provision device, and the master brake cylinder is permanently separated from the at least four wheel brakes.

10. The method as claimed in claim 9 , wherein the master brake cylinder is separated from the at least four wheel brakes by switching an isolating valve.

11. The method as claimed in claim 9 , wherein each wheel brake among the at least four wheel brakes is separably connected to the pressure medium reservoir tank via an outlet valve, and each outlet valve is permanently closed in the second fallback operating mode.

12. The method as claimed in claim 9 , wherein the monitoring comprises continuously monitoring values of the level of the pressure medium.

13. The method as claimed in claim 1 , wherein the pressure medium reservoir tank is under atmospheric pressure.

14. A brake system comprising:

a master brake cylinder actuatable by a brake pedal;

an electrically controllable pressure provision device;

a pressure medium reservoir tank, which is under atmospheric pressure and from which the master brake cylinder and the pressure provision device are supplied with pressure medium;

a sensor for determining a level of the pressure medium;

at least two hydraulically actuatable wheel brakes;

at least one electrically actuatable inlet valve per wheel brake; and

a control unit which controls the operation of the brake system,

wherein the wheel brakes are actuatable either with pressure medium from the master brake cylinder or with pressure medium from the pressure provision device, and

wherein the control unit is configured to control to close inlet valves of a first wheel brake group among the at least one electrically actuatable inlet valve when the level falls below a first limit value, and open the inlet valves of the first wheel brake group and close inlet valves of the second wheel brake group of the at least one electrically actuatable inlet valve when the level falls below a second limit value, which is lower than the first limit value.

15. The brake system as claimed in claim 14 , wherein the at least two hydraulically actuatable wheel brakes comprises at least four wheel brakes, and

wherein the first wheel brake group comprises a first wheel brake and a second wheel brake of the at least four wheel brakes.

16. The brake system as claimed in claim 14 , wherein the master brake cylinder is separably connected via an isolating valve to a brake supply line, to which the at least four wheel brakes are connected and which is separably connected via an activation valve to the pressure provision device.

17. The brake system as claimed in claim 16 , wherein the first wheel brake group comprises a first wheel brake and a second wheel brake of the at least four wheel brakes.

18. The brake system as claimed in claim 14 , wherein the master brake cylinder comprises a single pressure chamber which is hydraulically connected to the at least four wheel brakes.