It doesn't matter which of the two ports on the firewall side you connect either of the lines to. They both function the same/are interchangeable.

Now, if your lines are all mixed up it's possible for a check valve the be installed backwards, in which case it wouldn't function as intended, but I don't understand that to be your question.

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1979 300D 040 Black on Black - 1985 300D Maaco job (sadly sprayed over 199 Black Pearl Metallic) on Palamino

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