A vehicle deceleration sensing type dual braking system brake fluid pressure control valve used to maintain even braking pressure in the event of failure of one of the braking systems and which automatically sets a crossover pressure in dependence upon the load on the vehicle. Two reducing valve mechanisms are arranged coaxially within a valve body, with two plungers extending outwardly from a hollow liquid-pressure-operated piston. A spring pushes the plungers apart. Another liquid-pressure-operated piston, disposed coaxially with the two plungers, receives the outward end of one of the pistons. A chamber on the outward side of this liquid-pressure-operated piston is communicated with an inertial value used to sense the deceleration of the vehicle.