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Hydraulic Brakes System Diagnosis

WARNING: This page is about a different car, the 2003 GMC Safari and 2003 Chevrolet Astro. However, it is still accessible from the selected car via links, so may be relevant.
  1. Inspect and adjust brake fluid level in brake master cylinder. Was brake fluid level low? If so, go to next step. If not, go to step  3.
  2. Inspect brake fluid for the following conditions, indicating brake fluid contamination:
    • Fluid separation (indicating 2 types of fluid are present).
      1. Swirled appearance - oil-based substance.
      2. Layered appearance - silicone-based substance.
    • Fluid discoloration.
      1. Cloudy appearance - moisture.
      2. Dark appearance - suspended particles in fluid (dirt, rust, corrosion or brake dust).
    Inspect master cylinder reservoir cap diaphragm and reservoir-to-master cylinder grommets for swelling, indicating fluid contamination. Do any of the conditions exist? If so, go to step  4. If not, go to step  5.
  3. Inspect brake fluid for the following conditions, indicating brake fluid contamination:
    • Fluid separation (indicating 2 types of fluid are present).
      1. Swirled appearance - oil-based substance.
      2. Layered appearance - silicone-based substance.
    • Fluid discoloration.
      1. Cloudy appearance - moisture.
      2. Dark appearance - suspended particles in fluid (dirt, rust, corrosion or brake dust).
    Inspect master cylinder reservoir cap diaphragm and reservoir-to-master cylinder grommets for swelling, indicating fluid contamination. Do any of the conditions exist? If so, go to next step. If not, go to step  11.
  4. Flush hydraulic brake system. See HYDRAULIC SYSTEM FLUSHING . If 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.
    • Dry brake pipes using non-lubricated, filtered air.
    • Clean out the hydraulic brake pipes using denatured alcohol (or equivalent).
    • Repair or replace ALL of the following components listed. Each component contains internal rubber seals/linings which have been contaminated.
      1. See MASTER CYLINDER  under OVERHAUL or MASTER CYLINDER  under REMOVAL & INSTALLATION.
      2. Clean brake master cylinder reservoir using denatured alcohol, then dry the reservoir using non-lubricated, filtered air, or if necessary, replace the brake master cylinder reservoir. Replace brake master cylinder reservoir cap diaphragm.
      3. Replace front or rear brake hose.
      4. See FRONT BRAKE CALIPER  under REMOVAL & INSTALLATION or BRAKE CALIPER  under OVERHAUL.
      5. See BRAKE CALIPER (REAR)  under REMOVAL & INSTALLATION.
      6. See BRAKE PRESSURE MODULATOR VALVE in ANTI-LOCK article.
    If brake fluid was NOT contaminated with an oil-based fluid, but WAS contaminated with water or dirt, rust, corrosion, and/or brake dust, replace brake master cylinder reservoir cap diaphragm which may have allowed moisture or dirt to enter the system. Refill and bleed the hydraulic brake system. See BLEEDING BRAKE SYSTEM . After repairs, go to step  8.
  5. Inspect the hydraulic brake system components for external fluid leaks. Repair or replace any of the components found to be leaking brake fluid. Bleed hydraulic brake system. See BLEEDING 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 and/or the combination valve 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  11.
  6. Was there air in the system at a bleeder valve location other than at the repair location, (except if the brake master cylinder and/or the combination valve was replaced)? If so, go to step  18. If not, go to next step.
  7. 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  9.
  8. Inspect the hydraulic function of the brake calipers for proper operation. Was the hydraulic function of the brake calipers operating properly? If so, go to step  20. If not, go to step  13.
  9. Was the flow of brake fluid restricted or uneven through front axle hydraulic components during the bleeding procedure? If so, go to step  12. If not, go to next step.
  10. Was the flow of brake fluid restricted or uneven through rear axle hydraulic components during the bleeding procedure? If so, go to step  16.
  11. Inspect the hydraulic function of the brake calipers for proper operation. Was the hydraulic function of the brake calipers operating properly? If so, go to step  14. If not, go to next step.
  12. Determine if the brake caliper is restricting the flow of brake fluid and/or not operating properly: Raise and support vehicle. Remove wheels. Open the suspected caliper bleeder valve. 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. Close the caliper bleeder valve. Was the flow of brake fluid unrestricted and did the caliper piston move freely? If so, go to step  16. If not, go to next step.
  13. Repair or replace any brake caliper that was not operating properly. Refer to the appropriate procedure(s). See BRAKE CALIPER (FRONT)  or BRAKE CALIPER (REAR)  under REMOVAL & INSTALLATION. After repairs, go to step  20.
  14. 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 BLEEDING BRAKE SYSTEM . Was there air in the system? If so, go to step  18. If not, go to next step.
  15. Was the flow of brake fluid unrestricted and even per axle during the bleeding procedure? If so, go to step  20. If not, go to next step.
  16. Inspect hydraulic brake pipes and flexible brake hoses for signs of a fluid restriction, such as being bent, kinked, pinched or damaged. Replace any of the hydraulic brake pipes and/or flexible brake hoses found to be bent, kinked, pinched or damaged. 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  20. If not, go to next step.
  17. Replace hydraulic brake combination valve. See COMBINATION VALVE  under REMOVAL & INSTALLATION. After completing replacement, go to step  20.
  18. 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 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  20. If not, go to next step.
  19. Inspect brake master cylinder for internal fluid leaks. See LEAK TEST  under TESTING. 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 no, return to symptom table.
  20. Install or connect components that were removed or disconnected during diagnosis.
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