Brake pressure control valve for tractor-trailer vehicles

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. Thus, it is possible to modulate a pressure with an initial relatively sharp rise which is followed by a less gradual increase. In order to have a substantially equal characteristic curve during brake pressure decrease as well as during brake pressure buildup, and in order to reduce the hysteresis of the brake pressure control valve, the auxiliary control chamber is no longer vented into the working chamber but is directly vented to the atmosphere. The fluid pressure supply of the inlet valve is separated from the fluid pressure exhaust of the outlet valve of the second combination inlet and outlet valve.