A load-controlled fluid pressure regulating valve device for tractor-trailer type vehicle brake systems which combines in a single housing a unitary piston valve assemblage capable of performing the functions of a relay valve device and a regulating valve device. The unitary valve assemblage is acted upon under normal circumstances by the brake valve control fluid pressure and is further arranged for direct actuation by an emergency piston in the event of fluid pressure loss in the trailer supply line due to uncoupling or to a broken pipe. The valve assemblage is adjustably positioned in accordance with the vehicle load condition such that the relay valve function reflects the load regulating control valve.