I am unable to find an answer to this in the archives and it is driving me nuts. 1992 500SEL, the vacuum pump under the rear seat burned out due to almost constant running due to a vacuum leak/problem. I have replaced the pump but the problem that caused it to fail is still there. Everything connected to the pump seems to work ok, but when the transmission is put into any drive position the pump runs constantly at 2-3 second intervals and never stops. Put the transmission back into neutral or park and the pump stops running. I have traced the white (clear) line to the distribution block beneath the passenger footwell, but there is where I am stumped. What part failure is causing this symptom and what does it take to fix it? I'm letting this wonderful car sit in the garage because I don't want to risk burning up this pump again...they haint cheap!

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1996 SL320 97K miles
1996 C220 130k miles
1992 500SEL 170k miles <---sold
1986 300E 216k miles <---sold