It's been a while, but I think I used 1/2" ID copper water line. The servo on my 79 300TD is a black box on the front of the passenger side near the wheel well. There are two heater lines going in the front and exiting the rear of the servo. These inlet and exit nozzles are down on the bottom of the box, so disconnect them one side at a time, and run the copper lines underneath the box. Attach the heater lines to the copper line, making sure you have sufficient insert depth to use a hose clamp and not have it blow off the copper line.
Next to the box, comming from the water pump is a third line, this you cut and insert your ball valve. I actually used a gate valve, which offers more flow control. Again using the copper tubing, make nipples for either side of the valve, insert and tighten the compresion fittings, insert the valve into your line, start it up and look for leaks. Like I said, this sure fixed my problem of intermitent/no heat. Also keep in mind that I'm no mechanic, this is your basic hillbilly fix. If some mechanical type out there knows why this is a bad idea, let us know.