Abstract: A dual circuit brake valve system comprises first and second brake circuits, first and second combined inlet and outlet valves through which first and second braking pressures respectively are delivered to said first and second brake circuits, and first and second pistons which actuate the first and second combined inlet and outlet valves respectively. The second piston is actuated by the first braking pressure which serves as a control pressure therefor. To achieve simultaneous actuation of the first and second combined inlet and outlet valves, a compression spring is installed between the first and second pistons and mechanically connects these to each other. Thereby, a pressure difference of .DELTA.p between the pressure in the first brake circuit and the pressure in the second brake circuit, which approaches zero, is achieved. In order to be able to increase the pressure difference .DELTA.p in the two brake circuits, a third piston is provided between the first and second pistons.

Abstract: This invention relates to a brake pressure control valve and, more particularly, to a tractor-trailer control valve which includes a differential pressure surface relay piston that on one hand is acted upon by the control pressure from a control chamber and on the other hand by modulated pressure from a working chamber. The brake pressure control valve includes a first combination inlet and outlet valve which works together with the relay piston whereby the working chamber can be pressurized and vented via the first inlet and outlet valves. An auxiliary control chamber which is defined by the differential surfaces of the relay piston is connectable either to the working chamber or to the control chamber via second inlet and outlet valves. The pressure control surface of the relay piston is located opposite the working surface that is acted upon by control pressure in the control chamber. The pressure control surface can be enlarged by the pressure surface of the auxiliary control chamber.