The small hose from the rad to the resevoir may have a lose inner liner that is seperated from the woven section. That will act like a check valve under pressure but appear to be clear when removed. A new hose is couple of buck or so. I had this problem last year, went nuts checking things. I got this tip on the board.

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1984 300SD turbo 126
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God made me an atheist and who am I to question His wisdom