The part they are talking about most likely is the vacuum proportioning valve MB# 123 070 00 46. MB list $143. Might be cheaper here.
It is a commonly misdiagnosed device but they do go bad. The replacement is very very easy. The adjustment is very, very easy for me. But a mess can be made if you don't have experience. The system runs on a fragile balance between a metered source and a controlled leak (the part in question).
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician