Radiator question?

[White70C20]

I have been dealing with a stubborn coolant leak ever since I bought the truck in October. It started with a bad t-stat housing. Then I had problems with a plug on the top of the new t-stat housing leaking. In the process of changing the housings I noticed that the radiator drained fairly quickly. Yesterday, I finally had time to fix the leaking plug. While draining the radiator, I noticed it was draining really slowly. It got to the point that it quit draining, but the radiator was still half full. I undid the plug on the housing, and coolant gushed out.
Has anyone had this happen, and what did you do to fix it?
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