I've been doing my own troubleshooting regarding this vacuum system.

I applied vacuum to the system and it didn't hold. I tested the source line going to the brake booster and it was giving out good vacuum. I tested downstream of the orifice and it was reading good vacuum levels. I tested the line down to the modulator and it held vacuum for a good, long while. I then tested the VCV by itself and it would not hold vacuum. So, I guess I'll have to order a new VCV sometime in the future.

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'81 MB 300SD, '82 MB 300D Turbo (sold/RIP), '04 Lincoln Town Car Ultimate

Sooner or later every car falls apart, ours does it later!
-German Narrator in a MB Promotion Film about the then brand new W123.