The spec is a source vacuum or 400mbar with a max leak of 6mbar/min (12inHG, 0.2inHg).

If your brakes and AC are working properly I'd check downstream of the vacuum source first.

With the engine running, disconnect the line that goes to the key switch and hook it directly to the stop valve. If the engine shuts down quickly, the problem is between the key switch and stop valve. If the engine shuts down as it does with the key, the problem is upstream of the key switch.

I think the key switch sources off a tee fitting in the big vacuum line to the brake booster.

Sixto
91 300SE
87 300SDL
83 300SD ... any takers at $1100?