[Chocaholic]

Managed to find the offending Solenoid Valves in a box buried under the shrubs on the side of the house. These are 20 year old valves that appear to have never been rebuilt. Worse? It seems they are Hydro Flow valves. An internet search turned up:

Hydro Flow was sold to Hardie about 15 years ago. They were sold to Immotrol about 8 years ago. Immotrol makes a version of that valve and a company called Century makes a rebuild kit that fits the newer version of that valve...supposedly works on these older valves too. Found a supplier in Texas that carries the rebuild kits and ordered three of them yesterday. New diaghrams and solenoids for about 60 bux shipped.

I know you'll ask, why not just replace the valves with new Rainbirds from Lowes? A matter of digging up the plumbing under the hedge and cutting/redoing the plumbing to install new valves. Probably 10+ hours of work in 90+ degree weather, vs 15 minutes on my knees swapping out solenoids and diaghrams. No brainer! Have replaced a couple of bad heads already, so hopefully this fix will have things settled for another 10 years.

Just FYI for those with older irrigation systems. 20 minutes on the web saved hundreds of dollars and 10's of hours work. The more I dig into this system, the easier it's becoming to get it dialed in. Thanks for the encouragement....