Modulator assemblies for vehicle hydraulic braking systems

A modulator assembly for a vehicle anti-skid braking system comprising a housing having an inlet port for connection to an hydraulic master cylinder, an outlet port for connection to the slave cylinder of a wheel brake, a chamber through which said inlet port communicates with said outlet port and having an effective volume which is variable and a bore communicating with said chamber, a control valve located between said inlet port and said outlet port, an expander piston working in said bore and having a free end portion of a first area less than a second total area thereof, said expander piston being movable between a first advanced position in which said effective volume is at a minimum and said control valve is open and a second retracted position in which said control valve is closed and said effective volume is larger, and modulator means responsive to a signal from means for sensing deceleration of a wheel braked by said brake for applying a bias force to said expander piston to determine the position thereof, said expander piston normally being disposed in said first position but movable into said second position by said modulator means when deceleration of said braked wheel exceeds a predetermined value, wherein said chamber comprises a first compartment, a second compartment spaced axially from said first compartment and located at the end of said first compartment which is remote from said control valve, and a shoulder located between said compartments, valve means directly operable by, and axially aligned with, said expander piston are provided for controlling communication between said first and second compartments, and said outlet port leads from said first compartment, said valve means comprising a seal sealingly engagable with both said free end portion and said shoulder when said valve means is in a closed position and said first area only is exposed to pressure in said first compartment, said free end portion being movable away from said seal to provide communication between said compartments when said expander piston is moved into said second position whereafter said second total area of said piston is exposed to pressure in both said first and second compartments.