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Idle on 1990 Buick Riviera

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I have a 1985 Buick Riviera. Everything but the transmission is new. When I slow down and come to a stop at a stop light, the engine tends to race and I have to ease off the brake otherwise it will stall. Why does this happen and what can be done to eliminate it?
(1) Answer
I would check your Brake booster. It may have a vacuum leak that is allowing your engine to race. When you take your foot off the gas, it closes the booster's valve and shuts off the vacuum leak and then your engine may idle more normally.