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Brake System Internal Leak

WARNING: This page is about a different car, the 2003 Buick Regal, 2003 Buick Century, 2002 Buick Regal, and 2002 Buick Century. However, it is still accessible from the selected car via links, so may be relevant.
NOTE: Numbers in parentheses are referencing components in illustration. See Fig 1 .
  1. Start the engine and allow it to idle.
  2. Apply light, steady pressure to the brake pedal. Observe both the brake pedal feel and travel. Release the brakes and turn the ignition off.
  3. If the brake pedal apply felt spongy, but the brake pedal travel was not excessive, perform the following steps:
    1. Inspect the brake system for external leaks. See BRAKE SYSTEM EXTERNAL LEAK  under SYSTEM TESTS.
    2. Pressure bleed the brake system in order to purge any air that may be trapped in the system. See PRESSURE BLEEDING  under BLEEDING BRAKE SYSTEM.
  4. If the brake pedal apply did not feel spongy, but the brake pedal travel was excessive, perform the following steps:
    1. Loosen the master cylinder-to-brake power booster mounting nuts.
    2. Carefully pull the master cylinder away from the brake power booster just enough to inspect the mounting surface of the master cylinder.
    3. Inspect the master cylinder mounting surface at the primary piston (2) for brake fluid leaks. See Fig 1 .
  5. If the master cylinder exhibits any leakage around the primary piston (2), then the primary piston primary seal (4) and/or secondary seal (3) is leaking and the master cylinder requires overhaul or replacement. See Fig 1 .
  6. If the master cylinder primary piston (2) does not exhibit any leakage, pressure bleed the brake system. See Fig 1 . See PRESSURE BLEEDING  under BLEEDING BRAKE SYSTEM.
  7. If the brake pedal apply did not feel spongy, and the brake pedal travel was initially steady and not excessive, but then gradually fell, then the master cylinder requires overhaul or replacement due to an internal leak past the secondary piston (1) from the secondary piston primary seal (6) or secondary seal (5). See Fig 1 .
  8. If the brake pedal apply did not feel spongy, and the brake pedal travel was initially steady and not excessive, then fell slightly, then became steady again, then the Brake Pressure Modulator Valve (BPMV) may be leaking internally, and may require replacement.
Fig 1: Cross-Sectional View Of Master Cylinder
G00284607Courtesy of GENERAL MOTORS CORP.
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