Most late-model vehicles use a vacuum booster to help apply the brakes. The booster is usually located behind the master cylinder on the firewall. A leak in the vent valve can cause a reduction in the performance of the booster and increase pedal travel. A manifold vacuum of 20 inHg or greater can be achieved during deceleration. The booster chambers can be evacuated and retained at this pressure by a properly operating check valve.
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