Heater valves

[ccohen5]

Does anyone know what exactly gives out on the heater valve?  Is it a seal,
presumably internal, metal fatigue?

I open my heater valve before I shut the engine down in order to equalize
the static pressure between the cool water inside the heater and the hot
stuff outside.

Not sure if it helps, but it used to prolong the life of heater valves aka
reserve radiator control, on my various Coopers.

Colin