The way you describe your symtoms, it saounds like the vacuum pump or vacuum pump check valve has failed. The check valve is in the fitting, screwed into the pump. It is what the big vacuum line screws to. If you pull the vacuum supply hose off and can see through the fitting the valve is inside the pump. The valve is MB# 004 997 18 72 and costs MB list $21.50.
If the guts are missing you should also remove the front cover to search for them.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician