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

1. A brake system for motor vehicles comprising:

a master brake cylinder which can be actuated by a brake pedal and comprises only one pressure chamber;

an electrically controllable pressure supply device;

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

at least four hydraulically actuatable wheel brakes, each of the wheel brakes connected to a respective inlet valve for controlling flow of the pressure medium to the wheel brake;

wherein the wheel brakes can optionally be actuated by the master brake cylinder or by the pressure supply device;

wherein the wheel brakes are divided into at least a first wheel brake group and a second wheel brake group, the first wheel brake group comprising a first and a second wheel brake, the second wheel brake group comprising a third and a fourth wheel brake,

wherein the pressure chamber of the master brake cylinder is separably connected via an isolating valve to a first brake circuit supply line to which the first wheel brake group is connected, wherein the isolating valve is the only valve between the master brake cylinder and the inlet valves of the first wheel brake group,

wherein the pressure supply device is connected via a sequence valve which is normally open to a second brake circuit supply line to which the second wheel brake group is connected, and

wherein the first brake circuit supply line is separably connected via a connecting valve to the second brake circuit supply line, the connecting valve configured to be normally closed, such that the pressure chamber of the master brake cylinder is disconnected from the second brake circuit supply line by closing of the connecting valve.

2. The brake system as claimed in claim 1 , wherein the first and the second wheel brakes are arranged on opposite sides of the vehicle.

3. The brake system as claimed in claim 1 , wherein the isolating valve is configured to be normally open.

4. The brake system as claimed in claim 1 , wherein the connecting valve is configured to remain closed when a pressure difference across the connecting valve is lower than a pressure which corresponds to a pressure of the wheel brakes when the wheel brakes are fully actuated.

5. The brake system as claimed in claim 1 , wherein the electrically controllable pressure supply device comprises only one pressure chamber.

6. A method for operating a brake system for motor vehicles, the brake system comprising:

a master brake cylinder which can be actuated by a brake pedal and comprises only one pressure chamber;

an electrically controllable pressure supply device;

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

at least two hydraulically actuatable wheel brakes,

wherein the wheel brakes can optionally be actuated by the master brake cylinder or by the pressure supply device;

wherein the pressure chamber of the master brake cylinder is separably connected via an isolating valve to a first brake circuit supply line to which at least one of the wheel brakes is connected,

wherein the wheel brakes are divided into at least a first wheel brake group and a second wheel brake group,

wherein the first wheel brake group is connected to the first brake circuit supply line, and the pressure supply device is connected via a sequence valve which is normally open to a second brake circuit supply line to which the second wheel brake group is connected, and

wherein the first brake circuit supply line is separably connected via a connecting valve to the second brake circuit supply line, the connecting valve configured to be normally closed, such that the pressure chamber of the master brake cylinder is disconnected from the second brake circuit supply line by closing of the connecting valve,

wherein the pressure medium reservoir is equipped with a device for determining a level of the pressure medium, and

wherein the method comprises:

operating the brake system in a fallback operating mode when the determined level falls below a predefined threshold value, wherein in the fallback operating mode, the first wheel brake group is actuated by the master brake cylinder and the second wheel brake group is actuated by the pressure supply device as part of a single braking operation.

7. The method as claimed in claim 6 , wherein the brake system can be operated in a normal operating mode in which the first wheel brake group and second wheel brake group are actuated by the pressure supply device, by the connecting valve being switched into an open state and the isolating valve being switched into the closed state, and that the brake system is operated in the normal operating mode when the determined level lies above the predefined threshold value.

8. The method as claimed in claim 6 , wherein in the fallback operating mode, the connecting valve is held closed.

9. The method as claimed in claim 6 , wherein when failure of at least one valve of the brake system is detected, the brake system is operated in the fallback operating mode.

10. The method as claimed in claim 6 , wherein the wheel brakes are each separably connected to the pressure medium reservoir via a respective outlet valve, via a common return line, and the outlet valves are held closed in the fallback operating mode.

11. The method as claimed in claim 7 , wherein in the fallback operating mode, the connecting valve is held closed.