There is a small cylindrical pump that serves the servo (that's a joke sonny!),
Anyway, try testing this as well.
When I had the servo replaced on my w123, I could not get hot
air. After a NEW one was installed, the heat came back on.
My pump was o.k.

Do a search on the archives, it is all there

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1979 300D 220 K miles
1995 C280 109 K miles
1992 Cadillac Eldorado Touring Coupe 57K miles SOLD
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1979 240D 140Kmiles (bought for parents) *SOLD.
SAN FRANCISCO/(*San Diego)
1989 300SE 148 K miles *SOLD