A vehicle comprising: a master cylinder having an input side configured to receive an input from a brake pedal and an output side configured to provide a master cylinder output, at least one braking circuit having at least one wheel cylinder and a brake pressure generator, including a strokable piston, separate from the brake pedal; a simulator circuit including a pedal feel simulator coupled to the master cylinder output side through a switchable simulator valve, the pedal feel simulator providing a reaction force to the brake pedal when the switchable simulator valve is in an open position; at least one normally-open isolation valve (52 1, 52 2) operable to close and isolate the at least one braking circuit from the master cylinder and the simulator circuit; a pressure sensor operable in at least one vehicle configuration to be in fluid communication with both the simulator circuit and the brake pressure generator; and a controller programmed to, at a designated diagnostic time when no input is received from the brake pedal, establish a diagnostic circuit connecting the simulator circuit with an outlet of the brake pressure generator, and to stroke the piston in an advancing, pressure-generating direction and then retract the piston while observing a resulting brake fluid pressure decrease with the pressure sensor during piston retraction, wherein the controller is further programmed to check whether the relationship between the observed brake fluid pressure decrease and the piston retraction is within a predetermined acceptable range for continued operation of a brake-by-wire vehicle braking mode in which the master cylinder is coupled to the simulator circuit and decoupled from the at least one braking circuit, while brake fluid pressure is generated solely by the brake pressure generator. a master cylinder having an input side configured to receive an input from a brake pedal and an output side configured to provide a master cylinder output, at least one braking circuit having at least one wheel cylinder and a brake pressure generator, including a strokable piston, separate from the brake pedal; a simulator circuit including a pedal feel simulator coupled to the master cylinder output side through a switchable simulator valve, the pedal feel simulator providing a reaction force to the brake pedal when the switchable simulator valve is in an open position; at least one normally-open isolation valve (52 1, 52 2) operable to close and isolate the at least one braking circuit from the master cylinder and the simulator circuit; a pressure sensor operable in at least one vehicle configuration to be in fluid communication with both the simulator circuit and the brake pressure generator; and a controller programmed to, at a designated diagnostic time when no input is received from the brake pedal, establish a diagnostic circuit connecting the simulator circuit with an outlet of the brake pressure generator, and to stroke the piston in an advancing, pressure-generating direction and then retract the piston while observing a resulting brake fluid pressure decrease with the pressure sensor during piston retraction, wherein the controller is further programmed to check whether the relationship between the observed brake fluid pressure decrease and the piston retraction is within a predetermined acceptable range for continued operation of a brake-by-wire vehicle braking mode in which the master cylinder is coupled to the simulator circuit and decoupled from the at least one braking circuit, while brake fluid pressure is generated solely by the brake pressure generator.