Get a MitiVac hand pump and attach to the supply line in the engine compartment. If the system won't hold vac, you need to fix the leak.
If it does, then you have low vac for another reason, and will have to find out why. Old hard rubber connectors are a typical problem -- they leak, and you have low vac in that line.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!