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I don't know what the vacuum should be in your vehicle, but assuming that the manual is correct, your thoughts and the other posts will put you on track to find a vacuum leak. However, before you start, there is something you should understand about manifold vacuum.
A vacuum leak can affect manifold vacuum, but not directly, in the way you might think.
Strong intake manifold vacuum is an indication that your idle mixture is near optimum (the word I am groping for here starts with stoi.... but I can't spell it) on all cylinders.
If your rpm is not limited by an over-rich or over-lean condition, or by uneven operation of cylinders, then the only thing limiting your rpm is the throttle plate, and the engine will pull a strong vacuum behind the throttle plate.
A vacuum leak will result in low manifold vacuum, not because it is simply leaking in air, but because it messes up the air fuel mixture and the uniform operation of your cylinders. That causes your engine to develop less power, so it barely keeps running at idle RPM. The throttle plate sees less 'pull' as it restrains your engine to idle RPM, and hence, less manifold vacuum is measured.
Incorrect timing at idle, or even an obstructed exhaust, can also reduce your manifold vacuum. Anything that restrains your engine at idle will show up as reduced manifold vacuum.
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