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From: Daven Anderson
Whoops,missed this posting until today,sorry! You are describing the vacuum reserve tank for the power brake booster. The presence of this is so that if the engine is not generating enough vacuum when the brakes are applied the booster can draw on the reserve tank. Vacuum reserve tanks are a common upgrade for hotrods with longer duration camshafts than stock (that have less vacuum than stock-cam engines),but there are many factory-fit reserve tanks like yours.