Fluid pressure braking system

Fluid pressure braking system for tractor-trailer combination vehicle comprising a fluid pressure source on said tractor, a trailer supply line and a trailer control line communicating a portion of the fluid pressure braking system on the tractor with a portion of the fluid pressure braking system on the trailer, means for controlling communication to the trailer control line from said fluid pressure source, said means for controlling communication to the trailer control line including relay valve means having a supply port communicated to said fluid pressure source, a delivery port communicated to said trailer control line, and a control port, first operator-actuated valve means for controlling communication between said pressure source and the control port of the relay valve means, said relay valve means responding to pressure at said control port to communicate the supply and delivery ports of the relay valve means to therby communicate the trailer control line to said fluid pressure source, one-way check valve means for permitting fluid communication from said fluid pressure source into said supply port of the relay valve means but preventing communication in the reverse direction, and second operator-actuated valve means for controlling communication to said trailer supply line, said means for controlling communication to the trailer control line including valve mechanism communicated to the control port of the relay valve means to terminate communication to said control port when the trailer supply line is vented.