Hi there,
If you go to the SL forum there is an advertisement for the unwired tools digital servo at the top of the page. It is worthwhile investing in, as it cures the servo problem forever, at least a lot longer than 3 years! My 450SL had the aluminum bodied servo in it, but the gears, etc above the thing were all corroded away. I installed the digital servo and am very happy with it.
The unwired tools people have some downloads available on their website that include vacuum diagrams, installation instructions, etc that you can look at before you buy.

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Richard Wooldridge
'01 ML320
'82 300D 4.3L V6/T700R4 conversion
'82 380SL, '86 560SL engine/trans. installed
'79 450SL, digital servo update
'75 280C