A back-to-back proportioning valve assembly for a cross-split hydraulic braking system having separate first and second hydraulic brake system paths, said first brake system path including a first master cylinder chamber, a left front brake actuator and a right rear brake actuator, said second brake system path including a second master cylinder chamber, a right front brake actuator and a left rear brake actuator, wherein said proportioning valve assembly comprises:
- a first housing defining first and second inlet openings for fluid communication with corresponding first and second master cylinder chambers, left and right front outlet openings for communication with corresponding left and right front brake actuators, and left and right rear outlet openings for fluid communication with corresponding left and right rear brake actuators;
- first fluid path means within said first housing for direct fluid communication between said first inlet opening and said left front outlet opening such that the inlet fluid pressure at the first inlet opening is substantially equal to the outlet fluid pressure at the left front outlet opening;
- second fluid path means within said first housing for direct fluid communication between said second inlet opening and said right front outlet opening such that the inlet fluid pressure at the second inlet opening is substantially equal to the outlet fluid pressure at the right front outlet opening;
- a first proportioning valve means disposed in said first housing in fluid communication between said first inlet opening and said right rear outlet opening for proportioning the inlet fluid pressure at said first inlet opening with the outlet fluid pressure at said right rear outlet opening such that the outlet fluid pressure increases at a lower rate than the inlet fluid pressure, said first proportioning valve means having a second housing with a substantially cylindrical first chamber therein, said cylindrical first chamber having a first inlet port connected to said first inlet opening, said cylindrical first chamber having a first outlet port connected to said right outlet openings, said first proportioning valve having a first poppet located in said cylindrical first chamber to define a first orifice between the first poppet and said first outlet opening;
- a second proportioning valve means disposed in said first housing in fluid communication between said second inlet opening and said left rear outlet opening for proportioning the inlet fluid pressure at said second inlet opening with the outlet fluid pressure at said left rear outlet opening such that the outlet fluid pressure increases at a lower rate than the inlet fluid pressure, said second proportioning valve means having a third housing with a substantially cylindrical second chamber therein, said cylindrical second chamber having a second inlet port connected to said second inlet opening, said cylindrical second chamber having a second outlet port connected to said left outlet opening, said second proportioning valve having a second poppet located in said cylindrical second chamber to define a second orifice between the second poppet and said second outlet opening;
- a first disc located between said first outlet opening and said first proportioning valve means, said first disc having a first projection that extends into said first outlet opening;
- a second disc located between said second outlet opening and said second proportioning valve means, said second disc having a second projection that extends into said second outlet openings;
- first resilient means for urging said first poppet into continual engagement with said first projection;
- second resilient means for urging said second poppet into continual engagement with said second projection;
- means for moving said second and third housing of said first and second proportioning valve means with respect to said first and second poppets to adjust the size of the first and second orifices to compensate for changes in the inlet fluid pressure received from the first and second master cylinder chambers;
- a first bypass means in said housing for bypassing said first proportioning valve means to provide direct fluid communication between said first inlet opening and said right rear outlet opening when a predetermined first pressure differential develops between the fluid in the first and second master cylinder chambers as presented to the first and second inlet openings, said bypass means allowing the respective pressures at said first inlet and right rear outlet opening to be substantially equal;
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- a second bypass means in said housing for bypassing said second proportioning valve means to provide direct fluid communication between said second inlet opening and said left rear outlet opening when a predetermined fluid pressure differential develops between the fluid in the first and second master cylinder chamber as presented to the first and second inlet openings, wherein respective pressures at said second inlet and left rear outlet openings are substantially equal.