1966 Crown Coupe, 2016 200 S AWD, 1962 Lark Daytona V8.
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Do you still have the old parts to compare? Does the new booster have an adjustable pushrod and did instructions come with it? Is the pushrod there? We are assuming that these are the correct booster and m/cyl for the car.
The hiss is vacuum release from the booster vent valve letting air into the atmospheric-side of the booster chamber. A light hiss may be heard as you depress the pedal. A loud hiss may be a defective (torn diaphragm?) booster and you are hearing a direct manifold vacuum leak.
The hiss is vacuum release from the booster vent valve letting air into the atmospheric-side of the booster chamber. A light hiss may be heard as you depress the pedal. A loud hiss may be a defective (torn diaphragm?) booster and you are hearing a direct manifold vacuum leak.