Abstract: A back-to-back proportioning valve assembly for a cross-split braking system is disclosed. The proportioning valve assembly comprises an assembly housing defining a substantially cylindrical bore extending therethrough. The bore defines first and second rear brake outlet openings. In the assembly housing are disposed a pair of inlet passages extending within the cylindrical bore connected with respective first and second master cylinder chambers. A pair of front brake outlet openings are also disposed in the assembly housing. First and second proportioning valves are positioned in back-to-back relationship within the cylindrical bore adjacent respective first and second rear brake outlet openings for proportioning the inlet pressure to the rear brake outlets. A pair of bypass channels, for bypassing each of the proportioning valves, are provided to be opened upon failure of one of the braking systems.