I don't know how to do pictures but maybe I can describe it. The valve I used was a Bendix 40 PSI automatic dash control valve. It comes bare so make sure you order the mounting nut, red octagon trailer knob, and the roll pin. I tied the supply port of the valve into the manifold that is mounted inside the cab at the center of the firewall. The output of the valve goes to a bulkhead fitting that I installed on the firewall just inboard of the horns. The line goes down the firewall in the engine compartment and then bends under the floor of the cab between the frame and the engine bellhousing. An 18" long air brake hose attaches there and the other end is clamped to the frame and attaches to a new piece of tubing that runs along the frame to the crossmember right at the back of the transfer case. The M818 is different than the M813 because it is a tractor and has the pogo stick for the trailer at the back of the cab in addition to the rear gladhands. I think the M813 would be easier to plumb. On the M818, a supply line comes from the air tanks on the drivers side and goes along the crossmember until it tees into the pogo stick. The supply line continues over to the passenger frame rail where it tees into the line that goes to the rear gladhand and goes forward to supply the front axle shift valve on the transmission. I removed the tee at the pogo stick and put a right angle fitting in its place so that the pogo stick gets its air supply from the passenger side of the truck. Where that line ties into the tee on the passenger side, I removed the trans supply line and made a new line that goes from the gladhand lines tee to the new air brake line that I added from the supply valve. I then made a short jumper line connected to the old supply line from the tanks over to an elbow connected to the old line that goes to the trans supply. I think on the M813 all you would have to do is disconnect the gladhand line tee on the passenger frame rail and put a right angle fitting from the crossover supply line to the trans air valve and then replace the tee with a union to connect the rear glad hand line to your new trailer valve line.
It sounds complicated, but really all you are doing is changing where the rear glad hand gets its supply air. Instead of picking it up from the air tanks, it is getting air from your new valve in the cab.
Make sure that the hose you are using is a DOT approved air brake line. Also, the 3/8 copper tube fittings are not normal hardware store fittings. Trucks use DOT approved fittings made by Bendix and they do not interchange with commercial fittings. All of this stuff is available from Fleetpride or any other heavy duty truck supply house.
If you are thinking about doing this, you might wait a week or two until I hook up the trailer and drag it around to test it. Without the trailer, everything works perfectly but you never know until you test everything together. I will be getting the trailer out of its storage yard in a week or so and will let you know.
good luck, ...
