If you get a separate length of vacuum line and attach it to the shut off valve/shutoff servo/gizmo you can simply suck on that line to see if it will shut off the Engine. If that does not shut the engine off then you have a the shut off valve/shutoff servo/gizmo problem.

Although as the other member said you should be getting more vacuum for the line going to the vacuum switch on the steering colum lock.

If you replace the shut off valve/shutoff servo/gizmo read up on doing that because there is 2 possible places to stick the bent tab into If it goes into the wrong place when you start the Engine it will run away/go to max fuel.
As a precaution most people like to be ready to block off the air inlet incase the engine runs away.

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84 300D, 82 Volvo 244Gl Diesel