A vehicle braking system is provided. In a preferred embodiment the system has an actuator frame with a bore fluidly connected to the wheel brake and to a master cylinder via a solenoid valve. A piston, mounted within the bore, is operationally associated with a non-rotative nut which is threadably engaged by a power screw. To maximize reaction speed, a high efficiency thread is utilized between the power screw and the non-rotative nut. Therefore, pressure within the actuator can back drive the piston. To prevent backdriving of the piston there is a spring locking mechanism.