RRK,
I was concerned about your plumbing, but went to CPPs site and looked at basic Proportioning Valve plumbing. You are plumbed correctly.
Proportioning valve diagram My next concern is this, and I do not know if this has anything to do with the problems you are having, everything I have read about using an adjustable proportioning valve states that the original factory valve must be removed from the system. Having two valves trying to do the job of one in this situation can be disasterous. If you want the adjustability of the manual valve, get rid of the factory style and use the required items to complete the system. That is residual valves and metering valve. If they are applicable to your system.
I think you are going down the wrong road with both valves installed in the system.
Lonnie
