For WIW when I first tried to correct hard shifting on my 83 , I checked all the vaccum lines for leaks and found one that went no where. Thinking I had found the problem, I plugged it. The results were a hard downshift and the described clunk. Thought for sure the rear end was going out, but after researching old threads I learned that the line I plugged was a vaccum vent. Unplugged the line and no more clunk. This vent line comes off of the vaccum control valve on the IP. Check to make sure no one made the mistake I made.

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a few mb diesels