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Also See: Vacuum Advance
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vacuum reservoir
Since twin Weber manifolds have no Plenum, there is no place to get a steady vacuum signal. The reservoir is usually connected to all four intake runners and acts as a Plenum for vacuum. Connect the brake booster and vacuum gauge to the reservoir.
Larger ones also function as a Vacuum Reserve Canister. So when pulling a long hill you don't run out of vacuum for the brakes. Because vacuum is not made at heavy throttle it stores a little extra as a reserve.
If it's multi carb then you need a vacuum manifold (a tube to each runner connected to single reservoir) . Connect the distributor to the reservoir.
Use a vacuum reservoir about the size of two golf balls