Anti-skid hydraulic braking systems for vehicles

An anti-skid hydraulic braking system for vehicles comprising a wheel brake, a source of hydraulic fluid under pressure for applying said brake, skid sensing means for emitting skid signals responsive to behaviour of said wheel when said brake is applied, a modulator assembly for modulating the supply of fluid from said source to said brake in response to said skid signals from said skid sensing means, said modulator assembly comprising a housing having a bore, a de-boost piston working in said bore, a first valve, means in said housing defining an expander chamber between an end of said de-boost piston and said first valve, said first valve being disposed between said source and said expander chamber, means in said housing defining a support chamber to which said de-boost piston is subjected, and a second valve for controlling application to said de-boost piston of support fluid in said support chamber to determine the relative position of said de-boost piston in said bore, said support fluid normally being adapted to provide a biassing force for holding said de-boost piston in an advanced position in which the effective volume of said expander piston is at a minimum value, and said first valve being movable between an open position at least when said de-boost piston is in said advanced position and a closed position when said de-boost piston is displaced from said advanced position and into a retracted position, and means responsive to a skid signal to manipulate said second valve in order to release said support fluid thereby reducing said biassing force whereafter said piston can move away from said advanced position and into said retracted position in which said effective volume of said expander chamber is increased thereby reducing said pressure of fluid applied to said brake, wherein pressure-responsive means for controlling operation of said first valve are adapted to open said first valve upon failure of said support fluid and irrespective of the position of said de-boost piston.