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Hey sar,
I have a Z11 3 group in my shop and it's a great machine....

If you are pulling green water out of the boiler you should probably open the boiler (and get a fresh gasket while you are at it) and clean it right out. It's probably really skanky in there. Be sure to check all the wiring connections for all the valves, our switches started acting up but we traced the wires and simply found a loose connection. But as far as the vapor lock issue, I couldn't tell you what that could be all about unless the heat exchanger had some sort of slow leak and managed to empty out while the machine sat idle. Otherwise, parts are readily available online for these machines. I'd love to have a second Z and redo both.

Good luck,
Tom