Idles fine until you tap brakes

VDART said

try disconnecting the booster-- plug the line & see what happens--- try it before the check valve & after the check valve---- if it works fine -- you then have narrowed your problem down, Lawrence

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I agree...plug all vacuum sources....you may not be creating enough vacuum to use the booster....things that change it are too big of a cam and poor idle mixture setting...put a vacuum guage on the intake manifold and see what kind of vacuum you are pulling at idle...I would think that you need 14 to 16 lbs of vacuum at idle to run that booster..