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Hydraulic Brake Diagnosis

WARNING: This page is about a different car, the 2003 Pontiac Aztek and 2003 Buick Rendezvous. However, it is still accessible from the selected car via links, so may be relevant.
  1. Were you sent here from a Symptom Test? If so, go to next step. If not, go to GENERAL DIAGNOSIS  under TROUBLE SHOOTING.
  2. Inspect and adjust the brake fluid level in the brake master cylinder. See MASTER CYLINDER RESERVOIR  under BLEEDING, FILLING & FLUSHING BRAKE SYSTEM. Was the brake fluid level low? If so, go to next step. If not, go to step  4.
  3. Inspect the brake fluid for the following conditions, indicating brake fluid contamination:

    Fluid separation, indicating 2 types of fluid are present

    • Swirled appearance - oil-based substance.
    • Layered appearance - silicone-based substance.

    Fluid discoloration

    • Cloudy appearance - moisture.
    • Dark appearance/suspended particles in fluid - dirt, rust, corrosion, brake dust.
    Inspect the master cylinder reservoir cap diaphragm and the reservoir-to-master cylinder grommets for swelling, indicating fluid contamination. Do any of the above conditions exist? If so, go to step  5. If not, go to step  6.
  4. Inspect the brake fluid for the following conditions, indicating brake fluid contamination:

    Fluid separation, indicating 2 types of fluid are present

    • Swirled appearance - oil-based substance.
    • Layered appearance - silicone-based substance.

    Fluid discoloration

    • Cloudy appearance - moisture.
    • Dark appearance/suspended particles in fluid - dirt, rust, corrosion, brake dust.
    Inspect the master cylinder reservoir cap diaphragm and the reservoir-to-master cylinder grommets for swelling, indicating fluid contamination. Do any of the above conditions exist? If so, go to next step. If not, go to step  12.
  5. Flush the hydraulic brake system. See HYDRAULIC BRAKE SYSTEM FLUSHING  under BLEEDING, FILLING & FLUSHING BRAKE SYSTEM. If the brake fluid WAS contaminated with an oil-based or a silicone-based fluid, indicated by fluid separation and/or a swollen master cylinder reservoir cap diaphragm and/or swollen reservoir-to-master cylinder grommets, perform the following steps. Refer to the procedures indicated:
    • Remove ALL of the following components listed. Each component contains internal rubber seals/linings which have been contaminated.
    • Clean out the hydraulic brake pipes using denatured alcohol, or equivalent.
    • Dry the brake pipes using non-lubricated, filtered air.
    • Repair or replace ALL of the following components listed. Each component contains internal rubber seals/linings which have been contaminated.
      1. MASTER CYLINDER  under OVERHAUL or MASTER CYLINDER  under REMOVAL & INSTALLATION.
      2. Brake master cylinder reservoir: Clean the brake master cylinder reservoir using denatured alcohol, or equivalent, then dry the reservoir using non-lubricated, filtered air, or if necessary, replace the brake master cylinder reservoir.
      3. Replace the brake master cylinder reservoir cap diaphragm.
      4. BRAKE HOSE  under REMOVAL & INSTALLATION.
      5. BRAKE CALIPER - FRONT  under OVERHAUL or BRAKE CALIPER  under REMOVAL & INSTALLATION.
      6. BRAKE CALIPER - REAR  under OVERHAUL or BRAKE CALIPER  under REMOVAL & INSTALLATION, if equipped.
      7. WHEEL CYLINDER  under REMOVAL & INSTALLATION, if equipped.
      8. Brake Pressure Modulator Valve (BPMV). For replacement, see appropriate ANTI-LOCK article.
      If the brake fluid was NOT contaminated with an oil-based fluid, but WAS contaminated with water or dirt, rust, corrosion, and/or brake dust, replace the brake master cylinder reservoir cap diaphragm. Refill and bleed the hydraulic brake system. See HYDRAULIC BRAKE SYSTEM BLEEDING  under BLEEDING, FILLING & FLUSHING BRAKE SYSTEM. After completing the operation and any required repairs and/or replacements, go to step  9.
    1. Inspect the following hydraulic brake system components for external fluid leaks. Repair or replace any of the components found to be leaking brake fluid. Refer to the appropriate procedures:
    2. If you repaired or replaced any of the brake system components listed, bleed the hydraulic brake system. See HYDRAULIC BRAKE SYSTEM BLEEDING  under BLEEDING, FILLING & FLUSHING BRAKE SYSTEM. While bleeding the hydraulic brake system, observe for the following conditions:
      • The presence of air in the system at a bleeder valve location other than at the repair location, except if the brake master cylinder was replaced.
      • An unrestricted and even flow of brake fluid per axle during the bleeding procedure.
    Did you find and correct a condition? If so, go to next step. If not, go to step  12.
  7. Was there air in the system at a bleeder valve location other than at the repair location, except if the brake master cylinder was replaced? If so, go to step  19. If not, go to next step.
  8. Was the flow of brake fluid unrestricted and even per axle during the bleeding procedure? If so, go to next step. If not, go to step  10.
  9. Inspect the hydraulic function of the brake calipers/wheel cylinders for proper operation. See HYDRAULIC BRAKE COMPONENT OPERATION VISUAL INSPECTION - DISC  or HYDRAULIC BRAKE COMPONENT OPERATION VISUAL INSPECTION - DRUM  under HYDRAULIC BRAKE SYSTEM in ADJUSTMENTS & INSPECTION. Was the hydraulic function of the brake calipers/wheel cylinders operating properly? If so, go to step  21. If not, go to step  14.
  10. Was the flow of brake fluid restricted or uneven through front axle hydraulic components during the bleeding procedure? If so, go to step  13. If not, go to next step.
  11. Was the flow of brake fluid restricted or uneven through rear axle hydraulic components during the bleeding procedure? If so, go to step  17.
  12. Inspect the hydraulic function of the brake calipers/wheel cylinders for proper operation. See HYDRAULIC BRAKE COMPONENT OPERATION VISUAL INSPECTION - DISC  or HYDRAULIC BRAKE COMPONENT OPERATION VISUAL INSPECTION - DRUM  under HYDRAULIC BRAKE SYSTEM in ADJUSTMENTS & INSPECTION. Was the hydraulic function of the brake calipers/wheel cylinders operating properly? If so, go to step  15. If not, go to next step.
  13. Determine if the brake caliper is restricting the flow of brake fluid and/or not operating properly:
    1. Raise and support the vehicle.
    2. Remove the tire and wheel assemblies.
    3. Open the suspected caliper bleeder valve.
    4. Using a large C-clamp, compress the caliper piston and observe for an unrestricted flow of brake fluid and for free movement of the caliper piston.
    5. Close the caliper bleeder valve.
    Was the flow of brake fluid unrestricted and did the caliper piston move freely? If so, go to step  17. If not, go to next step.
  14. Repair or replace any brake caliper/wheel cylinder that was not operating properly. Refer to the appropriate procedure. After completing the repair and/or replacement, go to step  21.
  15. Bleed the hydraulic brake system to observe for the presence of air in the system and to observe for an unrestricted and even flow of brake fluid per axle during the bleeding procedure. See HYDRAULIC BRAKE SYSTEM BLEEDING  under BLEEDING, FILLING & FLUSHING BRAKE SYSTEM. Was there air in the system? If so, go to step  19. If not, the go to next step.
  16. Was the flow of brake fluid unrestricted and even per axle during the bleeding procedure? If so, go to step  21. If not, go to next step.
  17. Inspect the hydraulic brake pipes and flexible brake hoses for signs of a fluid restriction; such as being bent, kinked, pinched or damaged. See BRAKE PIPE & HOSE INSPECTION  under HYDRAULIC BRAKE SYSTEM in ADJUSTMENTS & INSPECTION. Replace any of the hydraulic brake pipes and/or flexible brake hoses found to be bent, kinked, pinched, or damaged. Refer to the following procedures as necessary: If none of the hydraulic brake pipes or flexible brake hoses were visibly bent, kinked, pinched, or damaged, replace the hydraulic brake flex hose at the restricted location. Did you find and correct a condition? If so, go to step  21. If not, go to next step.
  18. Replace the Brake Pressure Modulator Valve (BPMV), in order to correct the hydraulic brake dynamic rear proportioning mechanical operation. See REMOVAL & INSTALLATION in appropriate ANTI-LOCK article. After completing the replacement, go to step  21.
  19. Inspect the hydraulic brake system components for brake fluid seepage at a seal and/or fitting location, which may have drawn air into the system. Inspect the hydraulic brake system components for evidence of a recent repair, which may have introduced air into the system. Repair or replace any of the components found to be installed incorrectly or seeping brake fluid. Did you find and correct a condition? If so, go to step  21. If not, go to next step.
  20. Inspect the brake master cylinder for internal fluid leaks. See BRAKE SYSTEM INTERNAL LEAK INSPECTION  under HYDRAULIC BRAKE SYSTEM in ADJUSTMENTS & INSPECTION. Repair or replace the brake master cylinder if it is found to be leaking brake fluid internally. See MASTER CYLINDER  under OVERHAUL or MASTER CYLINDER  under REMOVAL & INSTALLATION.

    Did you find and correct a condition? If so, go to next step. If not, go to SYMPTOM LIST  under GENERAL DIAGNOSIS in TROUBLE SHOOTING.

  21. Install or connect components that were removed or disconnected during diagnosis. After completing the operation, the system should be okay.
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