A vehicle braking system comprising: 1.1 a first hydraulic braking circuit including a first hydraulic wheel cylinder (38a, 38b) operable to brake a first wheel (22a, 22b); 1.2 a second hydraulic braking circuit including a second hydraulic wheel cylinder (68a, 68b) operable to brake a second wheel (26a, 26b); and 1.3 a master cylinder having first and second outlets (21a, 21b), first and second pistons (140, 142) associated with the first and second outlets (21a, 21b), a fluid reservoir and a master cylinder input member provided with a mechanical coupling to a driver input member to drive the first piston from force applied to the driver input member and configured to drive the second piston only through movement of hydraulic fluid; 1.4 wherein the second hydraulic braking circuit is selectively disengageable from the master cylinder via a separation valve, the second hydraulic braking circuit including a pump and a line (82, 88) coupling a suction side of the pump with the master cylinder fluid reservoir, wherein a normally-closed variably adjustable pressure control valve is configured to modulate the pressure generated in the second hydraulic braking circuit, opening of the pressure control valve establishing a bridge whereby a discharge side of the pump has fluid communication with the line (82, 88), 1.5 the second hydraulic braking circuit is coupled to the first outlet of the master cylinder, and 1.6 the second hydraulic braking circuit has a first configuration in which an input force provided by a vehicle operator is applied to the first piston and carried through the first master cylinder outlet to apply a hydraulic braking force at the second hydraulic wheel cylinder (68a, 68b), and has a second configuration in which the pump of the second hydraulic braking circuit pumps hydraulic fluid from the fluid reservoir through the separation valve directly into the first master cylinder outlet between the first and second pistons (140, 142), causing the second piston to move farther from the first piston, wherein 1.7.1 the first hydraulic braking circuit includes a pump having fluid communication with the second master cylinder outlet through a line connecting the discharge side of the pump with the second master cylinder outlet; wherein 1.7.2 the first hydraulic braking circuit is a non-disengageable braking circuit in which a driver-initiated braking input is always carried through the second master cylinder outlet to apply a hydraulic braking force at the first hydraulic wheel cylinder (38a, 38b). 1.1 a first hydraulic braking circuit including a first hydraulic wheel cylinder (38a, 38b) operable to brake a first wheel (22a, 22b); 1.2 a second hydraulic braking circuit including a second hydraulic wheel cylinder (68a, 68b) operable to brake a second wheel (26a, 26b); and 1.3 a master cylinder having first and second outlets (21a, 21b), first and second pistons (140, 142) associated with the first and second outlets (21a, 21b), a fluid reservoir and a master cylinder input member provided with a mechanical coupling to a driver input member to drive the first piston from force applied to the driver input member and configured to drive the second piston only through movement of hydraulic fluid; 1.4 wherein the second hydraulic braking circuit is selectively disengageable from the master cylinder via a separation valve, the second hydraulic braking circuit including a pump and a line (82, 88) coupling a suction side of the pump with the master cylinder fluid reservoir, wherein a normally-closed variably adjustable pressure control valve is configured to modulate the pressure generated in the second hydraulic braking circuit, opening of the pressure control valve establishing a bridge whereby a discharge side of the pump has fluid communication with the line (82, 88), 1.5 the second hydraulic braking circuit is coupled to the first outlet of the master cylinder, and 1.6 the second hydraulic braking circuit has a first configuration in which an input force provided by a vehicle operator is applied to the first piston and carried through the first master cylinder outlet to apply a hydraulic braking force at the second hydraulic wheel cylinder (68a, 68b), and has a second configuration in which the pump of the second hydraulic braking circuit pumps hydraulic fluid from the fluid reservoir through the separation valve directly into the first master cylinder outlet between the first and second pistons (140, 142), causing the second piston to move farther from the first piston, wherein 1.7.1 the first hydraulic braking circuit includes a pump having fluid communication with the second master cylinder outlet through a line connecting the discharge side of the pump with the second master cylinder outlet; wherein 1.7.2 the first hydraulic braking circuit is a non-disengageable braking circuit in which a driver-initiated braking input is always carried through the second master cylinder outlet to apply a hydraulic braking force at the first hydraulic wheel cylinder (38a, 38b).