Brake System Internal Leak Test
WARNING: This page does not describe the selected car, but rather 12 other vehicles, including the 2003 GMC Yukon XL, 2003 GMC Yukon, 2003 GMC Sierra, 2003 GMC Cab & Chassis Sierra, and 2003 Chevrolet Tahoe. However, it is still accessible from the selected car via links, so may be relevant.
WARNING:
Brake fluid may irritate eyes and skin. In case of eye contact, rinse thoroughly with water. In case of skin contact, wash with soap and water. In case of ingestion, consult a physician immediately.
CAUTION:
Avoid spilling brake fluid onto painted surfaces, electrical connections, wiring, or cables. Brake fluid will damage painted surfaces and cause corrosion to electrical components. If any brake fluid comes in contact with painted surfaces, immediately flush the area with water. If any brake fluid comes in contact with electrical connections, wiring, or cables, use a clean shop cloth to wipe away the fluid.
- Start the engine and allow it to idle.
- Apply light, steady pressure to the brake pedal. Observe both the brake pedal feel and travel.
- Release the brakes and turn off the ignition.
- If the brake pedal apply felt spongy, but the brake pedal travel was not excessive, perform the following steps:
- Inspect the brake system for external leaks. See BRAKE SYSTEM EXTERNAL LEAK INSPECTION under ADJUSTMENTS & INSPECTION.
- Pressure bleed the brake system in order to purge any air that may be trapped in the system. See MANUAL BLEEDING or PRESSURE BLEEDING under BLEEDING BRAKE SYSTEM.
- If the brake pedal apply did not feel spongy, but the brake pedal travel was excessive, loosen the master cylinder-to-brake power booster mounting nuts. Carefully pull the master cylinder away from the brake power booster just enough to inspect the mounting surface of the master cylinder. Inspect the master cylinder mounting surface at the primary piston for brake fluid leaks. See Fig 1.
- 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.
- For overhaul, see MASTER CYLINDER under OVERHAUL.
- For replacement, see MASTER CYLINDER under REMOVAL & INSTALLATION.
- If the master cylinder primary piston does not exhibit any leakage, pressure bleed the brake system. See MANUAL BLEEDING or PRESSURE BLEEDING under BLEEDING BRAKE SYSTEM.
- 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.
- 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.