My experience with the MB version is nil, but from other brands experience says that the most common reason for pump failure is VALVE failure - i.e. hot gases backflowing through the pump. Check the valve - whether a check valve or vacuum-operated - before leaving the pump outlet port hose hanging free.

Steve

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'91 MB 190E 2.3
'08 RAV4 Ltd 3.5
'83 Lazy Daze m'home 5.7