Hi Is that a Rochester carb?? The middle port on a '76 is about 3/8" diameter, and goes to the power brake booster. For reference, a small port,( on the passenger side of the car) is for the transmission modulator line, and the one on the driver side is for a vacuum break canister on the carb. That one is a small port also. This is from a diagram in the '76 shop manual. Hope it helps.