A modulator assembly for a vehicle braking system includes a valve means for isolating a master cylinder from a brake actuator when a wheel slip condition arises at a braked wheel. Pressure modifying means is provided in the form of a piston which is moved axially by rotation of a motor armature 11 via a screw threaded connection to modify the braking pressure to correct the slip condition. Axial movement of the armature 11 moves a valve stem axially to close the valve means.