1982 300DT vacuum troubleshooting

I recently purchased the Vacuum Troubleshooting kit and have begun to tackle some minor issues with my wagon.
Right off the bat I find something not specified in your manual.
I started with a vacuum pump test and my gauge is reading 16.5 inHG. When I remove the line from the booster it reads 19 inHG - neither seems REALLY low, but are below the 20-23 inch range.

I have two questions:

1. Is there a way to determine if my checkvalve is leaking versus a slow leak in the pump itself?

2. The booster seems to be sucking a few inches of pressure off the line - is 2.5 inches of vacuum difference significant enough to need to replace the expensive booster?

Thanks for any wisdom you can share!

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