This is a very complicated system on these model diesels.
The system can not be diagnosed without a vacuum schematic. There are as many vacuum lines on these systems as electrical wires on early computer systems.
The only close to simple answer is to identify whether you have vacuum to the modulator. The modulator is on the drivers front of the trans behind the bell housing. Follow the line to the vac amplifier. This would be the easiest place to tee in. The vacuum should be high at idle and proportionally deminish as the throttle is engaged.
Remember that the source of vacuum is the brake booster supply line. If you have no vacuum at the modulator then I would check the source and follow the circuit diagram. Email me if you would like a diagram. Aside from finding a loose line I would advise an experienced tech to diagnose and adjust this sytem; there are too many interrelated adjustments to get skilled at this quickly.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician