that sounds like bad delivery valve seals. it would make sense with everything else you've described so far. I had the exact same problem with my 190 (parking on an uphill slope, etc.) and new delivery valve seals (and crush washers) was the cure.

hope that helps,

dave

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1987 190d turbo, 5-spd conversion -- 146k
1973 280 -- back on the road with a euro m110
1980 300SD -- RIP - Sold for parts (lost battle with rust)