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Brake System Internal Leak Test

WARNING: This page is about a different car, the 2003 GMC Sonoma, 2003 GMC Jimmy, 2003 Chevrolet S10 Pickup, and 2003 Chevrolet Blazer. However, it is still accessible from the selected car via links, so may be relevant.
  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.
  3. Release the brakes and turn ignition off.
  4. 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 INSPECTION  under ADJUSTMENT & INSPECTION.
    2. Pressure bleed the brake system in order to purge any air that may be trapped in the system. See HYDRAULIC BRAKE SYSTEM BLEEDING - MANUAL  or HYDRAULIC BRAKE SYSTEM BLEEDING - PRESSURE  under BLEEDING.
  5. 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 for brake fluid leaks.
  6. If the master cylinder exhibits any leakage around the primary piston, then the primary piston primary seal and/or secondary seal is leaking and the master cylinder requires overhaul or replacement. See MASTER CYLINDER  under REMOVAL & INSTALLATION.
  7. If the master cylinder primary piston does not exhibit any leakage, pressure bleed the brake system. See HYDRAULIC BRAKE SYSTEM BLEEDING - MANUAL  or HYDRAULIC BRAKE SYSTEM BLEEDING - PRESSURE  under BLEEDING.
  8. 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 from the secondary piston primary seal or secondary seal. See MASTER CYLINDER  under OVERHAUL or MASTER CYLINDER  under REMOVAL & INSTALLATION.
  9. 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. See BRAKE PRESSURE MODULATOR VALVE under REMOVAL & INSTALLATION in appropriate ANTI-LOCK article.
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