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Hydraulic Brake System Diagnosis

  1. If were you sent here from a brake symptom test, go to step  2. If you were not sent here from a brake symptom test, see HYDRAULIC BRAKE SYSTEM  under GENERAL DIAGNOSIS.
  2. Inspect and adjust the brake fluid level in the brake master cylinder. See MASTER CYLINDER RESERVOIR FILLING  under BLEEDING BRAKE SYSTEM. If the brake fluid level was low, go to step  3. If the brake fluid level was not low, go to step  4.
  3. Inspect the brake fluid for the following conditions, indicating brake fluid contamination:
    1. Fluid separation, indicating 2 types of fluid are present:
      • Swirled appearance, oil-based substance.
      • Layered appearance, silicone-based substance.
    2. Fluid discoloration:
      • Cloudy appearance, moisture.
      • Dark appearance/suspended particles in fluid, dirt, rust, corrosion and brake dust.
    Inspect the master cylinder reservoir cap diaphragm and the reservoir-to-master cylinder grommets for swelling, indicating fluid contamination. If any of the above conditions exist, go to step  5. If any of the above conditions DO NOT exist, go to step  6.
  4. Inspect the brake fluid for the following conditions, indicating brake fluid contamination:
    1. Fluid separation, indicating 2 types of fluid are present:
      • Swirled appearance, oil-based substance.
      • Layered appearance, silicone-based substance.
    2. Fluid discoloration:
      • Cloudy appearance, moisture.
      • Dark appearance/suspended particles in fluid, dirt, rust, corrosion and brake dust.
    Inspect the master cylinder reservoir cap diaphragm and the reservoir-to-master cylinder grommets for swelling, indicating fluid contamination. If any of the above conditions exist, go to step  5. If any of the above conditions DO NOT exist, 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:
    • 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. Perform master cylinder overhaul and/or master cylinder replacement. See MASTER CYLINDER  under OVERHAUL; or MASTER CYLINDER  under REMOVAL & INSTALLATION.
      2. 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. See MASTER CYLINDER RESERVOIR  under REMOVAL & INSTALLATION.
      3. Replace the brake master cylinder reservoir cap diaphragm.
      4. Replace front and/or rear brake hose. See BRAKE HOSE  under REMOVAL & INSTALLATION.
      5. Perform front and/or rear brake caliper overhaul. See BRAKE CALIPER  under OVERHAUL. Replace front and/or rear brake caliper. See BRAKE CALIPER  under REMOVAL & INSTALLATION.
      6. Replace Brake Pressure Modulator Valve (BPMV). 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 which may have allowed moisture or dirt to enter the system. Refill and bleed the hydraulic brake system. See BLEEDING BRAKE SYSTEM . If the operation and any required repairs and/or replacements were completed, 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:
      • Perform master cylinder overhaul and/or master cylinder replacement. See MASTER CYLINDER  under OVERHAUL; or MASTER CYLINDER  under REMOVAL & INSTALLATION.
      • Replace front and/or rear brake hose. See BRAKE HOSE  under REMOVAL & INSTALLATION.
      • Replace Brake Pipe. See BRAKE PIPE  under REMOVAL & INSTALLATION.
      • Perform front and/or rear brake caliper overhaul. See BRAKE CALIPER  under OVERHAUL. Replace front and/or rear brake caliper. See BRAKE CALIPER  under REMOVAL & INSTALLATION.
      • Replace Brake Pressure Modulator Valve (BPMV). See BRAKE PRESSURE MODULATOR VALVE in appropriate ANTI-LOCK article.
    2. If you repaired or replaced any of the brake system components listed, bleed the 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 was replaced.
      • An unrestricted and even flow of brake fluid per axle during the bleeding procedure.
    If the condition was found and corrected, go to step  7. If the condition was not found and corrected, go to step  12.
  7. If there was air in the system at a bleeder valve location other than at the repair location, except if the brake master cylinder was replaced, go to step  19. If there was nor air in the system at a bleeder valve location other than at the repair location, except if the brake master cylinder was replaced, go to step  8.
  8. If the flow of brake fluid was unrestricted and even per axle during the bleeding procedure, go to step  9. If the flow of brake fluid was not unrestricted and even per axle during the bleeding procedure, 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. If the hydraulic function of the brake calipers was operating properly, go to step  21. If the hydraulic function of the brake calipers was not operating properly, go to step  14.
  10. If the flow of brake fluid was restricted or uneven through front axle hydraulic components during the bleeding procedure, go to step  13. If the flow of brake fluid was not restricted or uneven through front axle hydraulic components during the bleeding procedure, go to step  11.
  11. If the flow of brake fluid was restricted or uneven through rear axle hydraulic components during the bleeding procedure, 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. If the hydraulic function of the brake calipers are operating properly, go to step  15. If the hydraulic function of the brake calipers are not operating properly, go to step  13.
  13. Determine if the brake caliper is restricting the flow of brake fluid and/or not operating properly:
    • Raise and support the vehicle.
    • Remove the tire and wheel assemblies.
    • 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.
    If the flow of brake fluid was unrestricted and if the caliper piston moved freely, go to step  17. If the flow of brake fluid was not unrestricted and if the caliper piston did not move freely, go to step  14.
  14. Repair or replace any brake caliper that was not operating properly. Refer to the appropriate procedure. Perform brake caliper overhaul. See BRAKE CALIPER  under OVERHAUL. If the repair and/or replacement was completed, 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 BLEEDING BRAKE SYSTEM . If there was air in the system, go to step  19. If there was no air in the system, go to step  16.
  16. If the flow of brake fluid was unrestricted and even per axle during the bleeding procedure, go to step  21. If the flow of brake fluid was not unrestricted and even per axle during the bleeding procedure, go to step  17.
    1. 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.
    2. 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:
      • Replace brake hose. See BRAKE HOSE  under REMOVAL & INSTALLATION.
      • Replace brake pipe. See BRAKE PIPE  under REMOVAL & INSTALLATION.
    3. 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.
    If the condition was found and corrected, go to step  21. If the condition was not found and corrected, go to step  18.
  18. Replace the brake pressure modulator valve (BPMV), in order to correct the hydraulic brake dynamic rear proportioning mechanical operation. See BRAKE PRESSURE MODULATOR VALVE in appropriate ANTI-LOCK article in BRAKES. After repairs are complete, go to step  21.
    1. 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.
    2. Inspect the hydraulic brake system components for evidence of a recent repair, which may have introduced air into the system.
    3. Repair or replace any of the components found to be installed incorrectly or seeping brake fluid.
    If the condition was found and corrected, go to step  21. If the condition was not found and corrected, go to step  20.
    1. Inspect the brake master cylinder for internal fluid leaks. See BRAKE SYSTEM INTERNAL LEAK TEST  under SYSTEM TESTS.
    2. Repair or replace the brake master cylinder if it is found to be leaking brake fluid internally. See MASTER CYLINDER  under OVERHAUL and/or MASTER CYLINDER  under REMOVAL & INSTALLATION.
    If the condition was found and corrected, go to step  21. If the condition was not found and corrected, return to symptom test.
  21. Install or connect components that were removed or disconnected during diagnosis. If the operation was completed, hydraulic brake system okay. If the operation was not completed, return to symptom tests.
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