overflowing from coolant expansion tank

I just went through a 2007 WR450F I purchased recently: suspension bearings, valve lash check, carb rejet, etc. Now back together, it is flowing water from the expansion tank overflow line.

 

I have the radiator overflow line routed into the bottom nipple of the expansion tank. The top nipple of the tank is the overflow hose, which is puking water.

 

I filled up the radiator past the fins to about the neck and I poured some water into the tank so it was about 1/3 full. Should I have left the tank empty?

 

During the restoration I pulled the water pump cover, replaced the o-ring, and checked the impeller for play.

 

The overflow water is warm/hot to the touch but not scalding. It is not boiling out of the radiator. I've tried two different radiator caps, a Mylers high pressure and the stock cap.

Edited by 2strokeYardSale

Make sure you have 50/50 coolant in there. It must be coming from the radiator to the overflow tank. If it's too full it will come out and level off.

Mine's an 03 but I filled my radiators all the way, my expansion tank maybe halfway, and I don't seem to be losing any. 

 

When you let yours cool back down, are the radiators still full?

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