Fill it through the expansion tank. If you want, you can also fill it through the thermostat housing and then top it off through the expansion tank. I think the second method makes it harder to get trapped air in the system. Make sure that it is properly bled of all air no matter what.

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Ali Al-Chalabi

2001 CLK55
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record