This can be caused by thing such very low or high pH, dissolved oxygen in your water, presence of sulfate or iron bacteria, high levels of total dissolved solids (salts), etc.
If you want more info go to http://www.fwr.org/copper.pdf
We had the same problem and replaced all of the pilot lines with stainless steel tubing and that solved the problem. If you do this you will have to replace the fittings to adapt to the stainless steel. In the future if you order valves you can spec them out to have stainless steel pilot lines.
Thanks these are both helpful. I wish i could figure out why it doesn't allow me to add pictures! Did you order stainless steel tubing from the manufacture or did bend it yourself? I have never worked with it, I am guessing it's not as easy to bend as copper.
Try reducing the size of the picture. If that does not work you can send it to me and I will upload it for you email@example.com
We also had this problem. We replaced all piloting tubing with stainless steel braided hoses with npt fittings and swivel ends. These work amazing. As well we keep a couple on each truck in case of leaks which we havent encountered yet. Easy to do, no bending or flaring involved.
Nuckfan.. I like the idea of the braided line and not having to bend. Would you be able to PM me some pictures of the connections you used?