Follow that vacuum line to the pump and you will see an adapter that connects the line to the pump. That is a check valve. They will come apart internally and cause this. You can replace the valve(after removal and taking out the parts that fell in with a magnet).

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Benzmac:
Donnie Drummonds
1991 GMC Syclone
ASE CERTIFIED MASTER AUTO TECHNICIAN
SERVICE MANAGER FOR 14 BAY FACILITY
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM