crusher

Have you already tried adjusting the Valve Cal per Ravens operators manual? Perhaps you've already been thru this book to no avail. I believe all of the C-FC valves must start the Valve cal w/a zero. Not sure which 440 you have, but this link takes you to the manual of the newer versions w/the single long character windows:

http://www.ravenprecision.com/Manuals/pdf/016-0159/822b.pdf

If that isn't your 440, you can find any of them on the Ravens site:

http://www.ravenprecision.com/us/Manuals/manualListing.jsp?ID=5&Type=13&SubType=0

I know this doesn't help your situation one bit, but we've run a FC Ravens valve for many years sidedressing thousands of acres of corn at rates similar to yours. For plot work I've cut back to as little as 30#/A with no problem, certainly no "hunting". Though I realize at some point of being almost closed, the control valve will probably cause fluctuations. You say you've ruined 3 valves? I don't think that's normal. I've only had one go bad on me but it was due to the NH3 that had vented off the pop-off valve of the Ravens cooler & blew right onto the FC valve case. When it quit we opened the case & most of the pc board had turned green. A deflector solves that problem (I guess). Like you, I do keep a spare valve in stock because most of our machine failures occur on Saturday afternoon around 1730 hrs. We like to keep going.

Perhaps the 2-valve system is better, we had that one many yrs ago also. It worked fine, but again, our FC valve works fine.

Again I'll ask, have you worked w/the VALVE CAL settings? Then of course, there is the CONTROL VALVE DELAY setting coming later in the setup. But, I don't think that setting has anything to do w/your problem.

Lastly (?), have you called Raven techs? They're good!!