The proportioning valve assembly may be screwed into a complementary-shaped opening in a master cylinder. The valve assembly includes a differential area piston biased by a spring toward the outlet which communicates with the rear brakes. The master cylinder includes a pair of passageways which communicate primary fluid pressure and secondary fluid pressure directly to the differential area piston of the proportioning valve assembly to displace the piston to a balanced position. If there is a failure of pressure in the primary brake circuit, the proportioning valve assembly actuates the differential area piston to permit fluid flow to bypass the piston and be communicated directly to the rear brakes of the vehicle. Upon failure, a differential area sleeve surrounding the piston moves to actuate an electrical failure warning switch.