Brake system , comprising • a first pressure supply unit , with an electromotive drive , which is configured to supply a pressure medium to at least one first brake circuit (BK1) and to at least one second brake circuit (BK2), • a motor-pump unit which is configured to supply the pressure medium to at least one of the brake circuits (BK1, BK2), • a second pressure supply unit which is configured to supply the pressure medium to least one of the brake circuits (BK1, BK2), wherein the second pressure supply unit is connected to the motor-pump unit via at least one first hydraulic line (HL1) and via at least one second hydraulic line (HL2), • a valve unit, wherein at least one of the brake circuits (BK1) is connected to the second pressure supply unit via at least one third hydraulic line (HL3), wherein the valve unit comprises at least one feed valve via which the third hydraulic line (HL3) can be at least partially reversibly shut off, wherein an isolating valve (BP1, BP2, TV1, TV2) is arranged in at least one of the hydraulic lines (HL1, HL2), via which the respective hydraulic line (HL1, HL2) can be at least partially reversibly shut off, characterized in that the second pressure supply unit comprises a main brake cylinder with exactly one piston and exactly one piston chamber , wherein the main brake cylinder comprises at least one expansion bore connected to a reservoir via a hydraulic line; and wherein the main brake cylinder comprises two ring seals , wherein at least one expansion bore is arranged between the ring seals , and wherein a throttle (DR) and preferably, a check valve (RV HZ ) connected in parallel, is arranged in the hydraulic line between the expansion bore arranged between the ring seals and the reservoir . • a first pressure supply unit , with an electromotive drive , which is configured to supply a pressure medium to at least one first brake circuit (BK1) and to at least one second brake circuit (BK2), • a motor-pump unit which is configured to supply the pressure medium to at least one of the brake circuits (BK1, BK2), • a second pressure supply unit which is configured to supply the pressure medium to least one of the brake circuits (BK1, BK2), wherein the second pressure supply unit is connected to the motor-pump unit via at least one first hydraulic line (HL1) and via at least one second hydraulic line (HL2), • a valve unit, wherein at least one of the brake circuits (BK1) is connected to the second pressure supply unit via at least one third hydraulic line (HL3), wherein the valve unit comprises at least one feed valve via which the third hydraulic line (HL3) can be at least partially reversibly shut off, wherein an isolating valve (BP1, BP2, TV1, TV2) is arranged in at least one of the hydraulic lines (HL1, HL2), via which the respective hydraulic line (HL1, HL2) can be at least partially reversibly shut off, characterized in that the second pressure supply unit comprises a main brake cylinder with exactly one piston and exactly one piston chamber , wherein the main brake cylinder comprises at least one expansion bore connected to a reservoir via a hydraulic line; and wherein the main brake cylinder comprises two ring seals , wherein at least one expansion bore is arranged between the ring seals , and wherein a throttle (DR) and preferably, a check valve (RV HZ ) connected in parallel, is arranged in the hydraulic line between the expansion bore arranged between the ring seals and the reservoir . <EOS>