Problem: Vehicles equipped with power brake booster assemblies fail.
Cause: 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.
Solution: Replace both master cylinder and power brake unit.
Note: When replacing brake components, always flush the entire brake system with approved brake fluid from a sealed container.
Courtesy of Cardone.