It seems that you and I have the exact same problem at the same time! Amazing , thanks for all the posts it has helped a lot, I now have second gear all the time and sometimes third! (when I started I had first and sometimes a slippy second). I can hear a leak inside when I release the accelerator so I will look around and see if I can find.

I just picked this car up yesterday and am truly amazed at the ludicrousness of what the vacuum system looked like when I got it. I wish I had taken a photo but lets just say that 80% was disconnected and then 30% of that was reconnected wrong and there was a lot of electrical tape involved.

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Thanks,
Will

1985 300D- 222k- (bad transmission)
1980 300D- (parts car)
1982 300SD- 242k