There is only one (1) line coming out of my vacuum pump, not two. I suppose the used pump that I just installed must be weak if the optimum vacuum should be 21 instead of 16 in/Hg, though I have read several posts that claim 10 in/Hg will work the shut-off valve okay. Should I try another vacuum pump R&R? Almost forgot to ask: Would a bad check valve at the vacuum pump cause my vacuum to be less than optimum?

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1980 300SD 146K miles.