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In a hydraulic anti-lock braking system a flow control valve operates with a single-acting solenoid operated valve (35) and the flew control valve incorporates a spool (17) working in a bore (8) in a housing (7) with the flow directed through a passage (20) passing substantially through the center o

In a hydraulic anti-lock braking system a flow control valve operates with a single-acting solenoid operated valve (35) and the flew control valve incorporates a spool (17) working in a bore (8) in a housing (7) with the flow directed through a passage (20) passing substantially through the center of the spool (7). A fixed orifice (21) is located at the one end of the spool which defines a chamber (19) for a control spring (18), and a variable orifice is defined between a port (33) in the housing (7) end a metering edge on the input side of an annular groove (24) in the spool (17) and which communicates with the passage (20) in the spool (17). A chamber containing the spring (18) being of a larger bore than the bore containing the spool (17).

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An hydraulic anti-lock braking system for a vehicle comprising a braked wheel, a speed sensor associated with said braked wheel for emitting speed signals, an electrical control unit for emitting control signals in response to said speed signals, means for applying pressure to said braked wheel unde

An hydraulic anti-lock braking system for a vehicle comprising a braked wheel, a speed sensor associated with said braked wheel for emitting speed signals, an electrical control unit for emitting control signals in response to said speed signals, means for applying pressure to said braked wheel under control of said signals from said control unit, a single solenoid-operated valve responsive to said control signals and movable between a closed brake-applying position and a open dump position, and a flow valve operable in response to operation of said solenoid-operated valve, wherein said flow valve comprises a housing, a sleeve having a bore defined by a first portion of smaller diameter, means defining a second bore portion of greater diameter, said sleeve being located in said housing and being provided with a number of cross passages, a spool working in said first bore portion in said sleeve and projecting into said second bore portion at a first end, said spool having a central axis and being provided with a number of external annular grooves and a first passage passing substantially through said central axis, a control spring, a portion of said second bore portion defining a spring chamber for said control spring, an expander chamber for fluid in communication with said spring chamber through said solenoid-operated valve, a fixed orifice located at said first end of said spool, a variable orifice defined between one of said cross passages in said sleeve and a motoring edge on an input side of one of said grooves in said spool and which communicates with said first passage in said spool, and dump means for releasing fluid from said brake, said dump means comprising one of said cross passages in said sleeve, one of said grooves in said spool adjacent to said first end in permanent communication with the said one of said cross passages, and a dump edge on an output side of the said one of said cross passages, said spool being movable between a brake-applying position and a dump position in response to a pressure drop across said fixed orifice upon movement of said solenoid-operated valve from said closed position into said open position, and wherein in said brake-applying position the said one cross passage is isolated from said spring chamber by co-operation of said spool with said first bore portion over an area of which an outer edge is defined by said dump edge, and in said dump position said one cross passage communicates with said expander chamber by flow through the said one groove, past said dump edge, through said spring chamber, and into said expander chamber through said open solenoid-operated valve.