I third the vacuum pump. '79 has the diaphgram type, if the diaphgram fails that's what happens. The clear tube is the most obvious, you can also open up any of the other vacuum lines and see if there's crank case oil in them. If that's the case, address as soon as possible. You really don't want oil in your vacuum lines.

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Sonny


86 190D N/A 2.5L Auto 265k "Ruby" -Sold-
79 300D 242k "Condi" my first -sold- RIP
2013 chevy sonic hatchback - had to for work