retx, a fellow member faxed me a service doc on this system. It recommends removing the servo to flush it and also allow the heater core to flush.

sorry just got in from out of town did not see your post sooner.

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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