I've replaced the core piece but not the entire valve. When I restart the hot engine, I get an initial blast that feels like full hot from the heater. This slowly tapers to cold but the wait is a good 5 to 10 minutes, not seconds.

Maybe I have a bad check valve. Is this part of the valve assembly or another item?

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Charles
1983 300D, bought new, 215k+ miles, donated to Purple Hearts veterans charity but I have parts for sale: http://www.peachparts.com/shopforum/showthread.php?t=296386