Radiator overflow tank

ir3333 said
you really need to be able to see the level to make sure it is working and continues to work.
When you shut your engine down the temperature rises immediately and your rad will overflow.As it cools it will draw it back in quite quickly.
I would never see it with an aluminum or stainless tank.

What you describe is "working" as a coolant recovery system, not a simple overflow catch-can.

I fill my radiator about 3/4" or 1" down from the neck, and little if any ends up in the tank. Especially since my fan controller has a cool-down timer after shutoff :)