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Hydraulic Brake System Diagnosis

WARNING: This page is about a different car, the 2002 GMC Sierra, 2002 GMC Cab & Chassis Sierra, 2002 Chevrolet Silverado, and 2002 Chevrolet Cab & Chassis Silverado. However, it is still accessible from the selected car via links, so may be relevant.
NOTE: This diagnostic table is designed to diagnose only the components of the hydraulic brake system in order to determine if the hydraulic brake system is operating properly.
  1. Were you sent here from a brake symptom? If so, proceed to next step. If not, see DIAGNOSTIC STARTING POINT  .
  2. Inspect and adjust the brake fluid level in the brake master cylinder. See MASTER CYLINDER RESERVOIR FILLING  under BLEEDING BRAKE SYSTEM. Was the brake fluid level low? If so, proceed 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 due to an oil-based substance.
    • Layered appearance due to a silicone-based substance.

    Fluid Discoloration

    • Cloudy appearance due to moisture.
    • Dark appearance and suspended particles in fluid due to dirt, rust, corrosion, or 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 due to an oil-based substance.
    • Layered appearance due to a silicone-based substance.

    Fluid Discoloration

    • Cloudy appearance due to moisture.
    • Dark appearance and suspended particles in fluid due to dirt, rust, corrosion, or 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, proceed to next step. If not, go to step  12 .
  5. Flush the hydraulic brake system. See HYDRAULIC BRAKE SYSTEM FLUSHING  under BLEEDING 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:
    1. Remove all of the following components listed. Each component contains internal rubber seals/linings which have been contaminated.
    2. Clean out the hydraulic brake pipes using denatured alcohol (or equivalent).
    3. Dry the brake pipes using non-lubricated, filtered air.
    4. Repair or replace all of the following components listed. Each component contains internal rubber seals/linings which have been contaminated.
      • See MASTER CYLINDER  under REMOVAL & INSTALLATION or see MASTER CYLINDER  under OVERHAUL. 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. Replace the brake master cylinder reservoir cap diaphragm.
      • See BRAKE HOSE  under REMOVAL & INSTALLATION.
      • See BRAKE CALIPER  under REMOVAL & INSTALLATION.
      • See BRAKE CALIPER  under OVERHAUL.
      • See COMBINATION VALVE  under REMOVAL & INSTALLATION.
      • See BRAKE PRESSURE MODULATOR VALVE under REMOVAL & INSTALLATION in 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 which may have allowed moisture or dirt to enter the system. Refill and bleed the hydraulic brake system. See MANUAL BLEEDING  or PRESSURE BLEEDING  under BLEEDING BRAKE SYSTEM. When complete, go to step  9 .
  6. 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 procedure(s): If you repaired or replaced any of the brake system components listed, bleed the hydraulic brake system. See MANUAL BLEEDING  or PRESSURE BLEEDING  under 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, proceed 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 and/or the combination valve was replaced)? If so, go to step  19 . If not, proceed to next step.
  8. Was the flow of brake fluid unrestricted and even per axle during the bleeding procedure? If so, proceed to next step. If not, go to step  10 .
  9. Inspect the hydraulic function of the brake calipers for proper operation. See HYDRAULIC BRAKE COMPONENT OPERATION VISUAL INSPECTION  under ADJUSTMENTS & INSPECTION. Was the hydraulic function of the brake calipers 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, proceed 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 for proper operation. See HYDRAULIC BRAKE COMPONENT OPERATION VISUAL INSPECTION  under ADJUSTMENTS & INSPECTION. Was the hydraulic function of the brake calipers operating properly? If so, go to step  15 . If not, proceed to next step.
  13. Determine if the brake caliper is restricting the flow of brake fluid and/or not operating properly:
    1. Raise and suitably 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, proceed to next step.
  14. Repair or replace any brake caliper that was not operating properly. See BRAKE CALIPER  under REMOVAL & INSTALLATION or BRAKE CALIPER  under OVERHAUL. When complete, 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 MANUAL BLEEDING  or PRESSURE BLEEDING  under BLEEDING BRAKE SYSTEM. Was there air in the system? If so, go to step  19 . If not, proceed 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, proceed 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 ADJUSTMENTS & INSPECTION. Replace any of the hydraulic brake pipes and/or flexible brake hoses found to be bent, kinked, pinched or damaged. See BRAKE HOSE  or BRAKE PIPE  under REMOVAL & INSTALLATION. If none of the hydraulic brake pipes or flexible brake hoses were visibly bent, kinked, pinched or damaged, replace the hydraulic brake flex hose(s) at the restricted location. Did you find and correct a condition? If so, go to step  21 . If not, proceed to next step.
  18. Replace the hydraulic brake combination valve. See COMBINATION VALVE  under REMOVAL & INSTALLATION. When complete, 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, proceed to next step.
  20. Inspect the brake master cylinder for internal fluid leaks. See BRAKE SYSTEM INTERNAL LEAK TEST  under SYSTEM TESTS. Repair or replace the brake master cylinder if it is found to be leaking brake fluid internally. See MASTER CYLINDER  under REMOVAL & INSTALLATION or see MASTER CYLINDER  under OVERHAUL. Did you find and correct a condition? If so, proceed to next step. If not, return to symptom or list that sent you to this procedure.
  21. Install or connect components that were removed or disconnected during diagnosis. When complete, hydraulic brake system is okay.
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