Brake master cylinder for a brake system of a vehicle having: a first pressure chamber, on which at least one first reservoir attachment opening is configured; a first piston component and a second piston component; it being possible for the at least one first reservoir attachment opening of the first pressure chamber to be sealed by means of the first piston component which is adjusted out of its first starting position at least by a first minimum sealing travel, it being possible for the at least one first reservoir attachment opening of the first pressure chamber to be additionally sealed by means of the second piston component which is adjusted out of its second starting position at least by a second minimum sealing travel, the first minimum sealing travel being greater than the second minimum sealing travel, the brake master cylinder having a second pressure chamber which is delimited by the second piston component and on which at least one second reservoir attachment opening is configured which can be sealed by means of the second piston component, and it being possible for a brake circuit attachment opening which is configured on the second pressure chamber in addition to the at least one second reservoir attachment opening to be closed by means of a valve of the brake master cylinder, characterized in that the valve is configured as a normally open valve, and the brake master cylinder can be controlled into a first operating mode by way of controlling of the valve into its closed state, in which first operating mode a first maximum of a counter-force which counteracts the adjustment of the first piston component along the first minimum sealing travel is smaller than a second maximum of a counter-force which counteracts the adjustment of the second piston component along the second minimum sealing travel, whereas the brake master cylinder can be controlled into a second operating mode by way of controlling of the valve into its at least partially open state, in which second operating mode the first maximum of the counter-force which counteracts the adjustment of the first piston component along the first minimum sealing travel is greater than or equal to the second maximum of the counter-force which counteracts the adjustment of the second piston component along the second minimum sealing travel. a first pressure chamber, on which at least one first reservoir attachment opening is configured; a first piston component and a second piston component; it being possible for the at least one first reservoir attachment opening of the first pressure chamber to be sealed by means of the first piston component which is adjusted out of its first starting position at least by a first minimum sealing travel, it being possible for the at least one first reservoir attachment opening of the first pressure chamber to be additionally sealed by means of the second piston component which is adjusted out of its second starting position at least by a second minimum sealing travel, the first minimum sealing travel being greater than the second minimum sealing travel, the brake master cylinder having a second pressure chamber which is delimited by the second piston component and on which at least one second reservoir attachment opening is configured which can be sealed by means of the second piston component, and it being possible for a brake circuit attachment opening which is configured on the second pressure chamber in addition to the at least one second reservoir attachment opening to be closed by means of a valve of the brake master cylinder, characterized in that the valve is configured as a normally open valve, and the brake master cylinder can be controlled into a first operating mode by way of controlling of the valve into its closed state, in which first operating mode a first maximum of a counter-force which counteracts the adjustment of the first piston component along the first minimum sealing travel is smaller than a second maximum of a counter-force which counteracts the adjustment of the second piston component along the second minimum sealing travel, whereas the brake master cylinder can be controlled into a second operating mode by way of controlling of the valve into its at least partially open state, in which second operating mode the first maximum of the counter-force which counteracts the adjustment of the first piston component along the first minimum sealing travel is greater than or equal to the second maximum of the counter-force which counteracts the adjustment of the second piston component along the second minimum sealing travel.