Sounds like a vacuum leak in the brake booster. They can't be rebuilt.Throwing kits in those carbs won't help much unless you know what you are doing. Those 35/40 Zeniths were excellent carburetors, but the problem is finding anyone still around who knows them. Check around to find a professional senior mechanic in you area who knows Zeniths. If someone tells you a carb is a carb, move on.
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