Abstract: A brake control valve device for a use with tandem or tractor-trailer type vehicles employing dual brake systems with the source of operating fluid supplied at a relatively higher pressure than that required for operating the brakes. The respective valve elements of the valve device for controlling supply and exhaust of operating pressure to and from the respective brake circuits are mechanically actuated through a manually actuated pressure graduating piston (for the first brake circuit) and a fluid pressure responsive relay piston (for the second brake circuit), the dimensions of the relay piston being limited to less than normal for conserving space, but being provided with an adjusting mechanism for adjusting the fluid pressure responsive operating range thereof for compensating for the smaller dimension and synchronizing the relative serial operation of the dual brake circuits.