My 1980 300D (which I have owned since 1984) has
had a nasty flare on 2-3 shift for more than a decade....
until now. It shifts better than it ever has after I replaced
the little valve on top of the engine with the vacuum lines running to it.
Unlike other models with a cover over the valve, my 300D has a very visible
valve and some vacuum lines.
I tested the valve with a vaccum pump (mighty-vac type) and compared the results to a brand new one I happened to have.
It was clear the old one leaked. The new valve is easily $30 to $45 and
can be bought here through Fastlane or from your dealer.
Originally I was going to only replace the worn out valve lever ( a $3 item),
but decided to check the valve while in there, really glad I did.
Here is my post on the subject :
2-3 Flare on Shift - replaced 3 way valve.