A vehicle braking system 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 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 2; 52 1) operable to close and isolate the at least one braking circuit from the master cylinder and the simulator circuit; a primary pressure sensor operable to generate a braking request signal responsive to the input from the brake pedal; and a controller programmed to activate the brake pressure generator to apply a braking force to the at least one wheel cylinder of the braking circuit based on the braking request signal from the primary pressure sensor, the controller further being programmed to close the at least one normally-open isolation valve (52 2; 52 1) during activation of the brake pressure generator so that the master cylinder is coupled to the simulator circuit and decoupled from the braking circuit; wherein the controller is further programmed to identify an abnormal value for the braking request signal from the primary pressure sensor, and in response, the controller is programmed to open the at least one normally-open isolation valve (52 2; 52 1), characterized in that the controller is further programmed to observe a relationship between brake pedal actuation and pressure generated at the master cylinder output side using a secondary pressure sensor that is positioned in the braking circuit such that the controller determines whether the abnormal value is indicative of a primary pressure sensor malfunction or indicative of a malfunction of at least one of the pedal feel simulator and the switchable simulator valve. 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 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 2; 52 1) operable to close and isolate the at least one braking circuit from the master cylinder and the simulator circuit; a primary pressure sensor operable to generate a braking request signal responsive to the input from the brake pedal; and a controller programmed to activate the brake pressure generator to apply a braking force to the at least one wheel cylinder of the braking circuit based on the braking request signal from the primary pressure sensor, the controller further being programmed to close the at least one normally-open isolation valve (52 2; 52 1) during activation of the brake pressure generator so that the master cylinder is coupled to the simulator circuit and decoupled from the braking circuit; wherein the controller is further programmed to identify an abnormal value for the braking request signal from the primary pressure sensor, and in response, the controller is programmed to open the at least one normally-open isolation valve (52 2; 52 1), characterized in that the controller is further programmed to observe a relationship between brake pedal actuation and pressure generated at the master cylinder output side using a secondary pressure sensor that is positioned in the braking circuit such that the controller determines whether the abnormal value is indicative of a primary pressure sensor malfunction or indicative of a malfunction of at least one of the pedal feel simulator and the switchable simulator valve.