if you are "Teed" into the Purge line , when the purge valve is duty cycled by the ECM / PCM
the vacuum gauge will pulse with the duty cycle .
17 to 18" at hot idle is too low at sea level
but more important is that at hot idle there should be ZERO flutter in the needle
the brake booster vacuum line has a check valve in it ,
if you "T" into the line after the check valve the vacuum value you measure will be incorrect.
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