I also had low vacuum problems that also included a leaking brake booster. Because of the booster leak the pump created 4" Hg less vacuum than when measured without the booster connected. A salvage yard booster set me back $75.

You can check this quickly by connecting a vacuum gauge (or mityvac) to the main vac line, disconnecting the hose to the booster, starting the car and putting you thumb over the hose you disconnected from the booster. Compare that vacuum reading (and the steadyness of the gauge) with the reading when the booster is connected.

Before changing the booster make one final check which is to test the booster alone with a Mityvac. This is done by somehow connecting your mityvac to only the booster and pumping it down. I would only be concerned about a gross leak since it won't effect the other components because of the CV. A gross leak like i had never allowed the vacuum level to go deep enough and CVs will not help that.

BTW, I hate the tapered end on my Mityvac. It is large diameter and damages the small MB vacuum hose ends. The Snap-on brand hand vacuum testing pump has a perfect dia for MB hoses but it's probably a lot more $ than the Mityvac. All the MB techs at the dealer I know only use Snap-on ones for this very reason.

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1972 450SL
1982 300D Turbo