I think the blank line is a vent line, sort of like a ground. I am pretty sure I read that on one of the other posts around here.

With regard to sucking oil through the vac system, I think that often the vacuum fuel shutoff will leak at the IP (which has motor oil circulating within). When that diaphram fails, oil can get sucked back toward the source of the vacuum. There are probably other ways to accomplish the same thing.

later

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