Where exactly did you plug into the vacuum system? You should T it off of the line from the manifold. The stock config has a line that goes from intake manifold to an Overboost protection valve on Firewall near the fusebox. Often times it is plugged up which will cause no boost or low boost. Bypassing it is safe...mine has been bypassed for 2 years.

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1987 300TDT - 195,000 (Original #14 head)
1993 190E 2.3 - 105,000
1981 300D - 250,000