It is a liquid and gas vapor trap. The 2 lines from the tank come from the high points of the tank on the left and right sides. Any gas vapor that expands out of the tank would first go there and then up to the charcoal canister where it is absorbed by the charcoal. When you are driving the vapor is then pulled into the carb and burned. There should be a hose that is T'd into you PCV line that goes to the canister.
This is why the 71+ gas caps are different than the 70 and older. 70 and older are vented and 71+ are sealed and usually say "Caution, Pressure-Vacuum" on them.(or something like that anyway) On the trucks there is a long loop of line that runs up into the cab roof on the rear passenger corner.
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1971 C-10 Suburban
1972 C-10 Suburban
1969 XL Galaxie Fastback
1968 Galaxie 500 Fastback
1974 SCOUT II
Montrose, Minnesota
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1971 C-10 Suburban (Ochre)
1971 K-10 Suburban (Ochre)
1972 C-10 Suburban (Ochre)
1972 K-20 Suburban (Yellow- that just aint right!)
Springfield, Minnesota 56087
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