Chronic master cylinder failure due to fluid leaking past secondary seal in the master cylinder bore. A defective master cylinder pushrod seal in the power brake booster allows vacuum to draw fluid past secondary seal of the master cylinder.
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.