What year and model are we talking about? On the 83 there is no pneumatic connection to the pod in the door but on later models there is and if pressure stays on the pod you will not be able to push it down. Try disconnecting the vac/pressure line and then push the knob down. Also it is my understanding that you are not supposed to be able to push the knob down when the door is open.

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1983 300SD
200000miles