Good question ! I wondered also because it seemed that if I put one in a spot where it might protect from oil contamination from faulty vacuum diaphram , it would be cheap insur. against extended vac. line contamination.(300D) Haven't done it yet but
this forum's PARTSSHOP does have them.
2 cents
any tech comments?

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1979 300D
133K miles
1989 300SE
1995 C280