Two things that typically fail. One, the monovalve like you have found. It sticks shut and will not allow the coolant to flow. The other is the auxiliary water pump the bearings are exposed to the atmosphere and everything it holds (salt) and it usually siezes up redenering you limited or no coolant to the heater core.

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2003 Dodge Ram 2500 Cummins - Georend fully built trans, Banks Big Hoss Bundle, Smarty Tuner, 5" turbo back exhaust. 500 HP and 1100 ft/lbs to the ground.
1979 Mercedes-Benz 300CD - Lost to a four car pile up on 275, Thank You Michigan Drivers.
1986 Mercedes-Benz 300D - Sold, Euro non-turbo car.