brake booster bad ?

One way to test your booster is with the engine off, pump the brake pedal to make sure you have no vacuum to assist you anymore. Then depressed the brake pedal with modest effort.

Then start your engine, the pedal should gently soften with the pressure your foot has on it. if it remains hard then your booster is not working. Also check the valve that is meant to hold vacuum in the booster. they can cause or be the leak too.

A leaking booster is a vacumm leak and a vacuum leak makes for poor idle qaulity and can behave like a miss also.