You may have a bad diaphram, and you may just have a serious leak in the main hose. Check the metal end for tightness, then all the plastic and rubber parts for integrity and tightness.
Locks will work on very low vacuum, usually, even if the shutoff and brake booster don't. For you to lose the booster indicates a very serious leak (as in big cracks in the main line) or a bad diaphram.
All the rubber in the engine compartment goes bad with time, heat, and oil, so expect to replace all the little rubber ends on the vacuum hoses.
If you hear a funny droning noise and have lots of blue smoke (and high oil consumption) the diaphram is definitly shot. There will also be oil in the pressure side line that goes to the intake manifold.
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!