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Hydraulic System Test

  1. If sent here from SYMPTOM INDEX  table, go to next step. If not sent here from SYMPTOM INDEX  table, perform HYDRAULIC BRAKE DIAGNOSTIC SYSTEM CHECK  under TROUBLE SHOOTING.
  2. Inspect and adjust the brake fluid level in the brake master cylinder. See MASTER CYLINDER RESERVOIR FILLING  under BLEEDING, FILLING & FLUSHING BRAKE SYSTEM. If brake fluid level is low, go to next step. If brake fluid level is not low, go to step  4.
  3. Perform the following:
    1. Inspect the brake fluid for the following conditions, indicating brake fluid contamination:
      • Fluid separation, indicating 2 types of fluid are present. A swirled appearance indicates an oil-based substance and a layered appearance indicates a silicone-based substance.
      • Fluid discoloration. A cloudy appearance indicates moisture and a dark appearance indicates suspended particles in fluid (dirt, rust, corrosion, brake dust).
    2. 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 the above conditions do not exist, go to step  6.
  4. Perform the following:
    1. Inspect the brake fluid for the following conditions, indicating brake fluid contamination:
      • Fluid separation, indicating 2 types of fluid are present. A swirled appearance indicates an oil-based substance and a layered appearance indicates a silicone-based substance.
      • Fluid discoloration. A cloudy appearance indicates moisture and a dark appearance indicates suspended particles in fluid (dirt, rust, corrosion, brake dust).
    2. 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 the above conditions do not exist, go to step  12.
  5. Perform the following:
    1. Flush the hydraulic brake system. See HYDRAULIC BRAKE SYSTEM FLUSHING  under BLEEDING, FILLING & FLUSHING BRAKE SYSTEM.
    2. 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:
      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.
        • Master cylinder. See MASTER CYLINDER  under OVERHAUL or MASTER CYLINDER  under REMOVAL & INSTALLATION. Also perform the following: 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.
        • Front brake hose. See FRONT BRAKE HOSE  under REMOVAL & INSTALLATION.
        • Rear brake hose. See REAR BRAKE HOSE  under REMOVAL & INSTALLATION.
        • Front brake caliper. See BRAKE CALIPER  under OVERHAUL orBRAKE CALIPER  under REMOVAL & INSTALLATION.
        • Wheel cylinder. See WHEEL CYLINDERS  under REMOVAL & INSTALLATION.
        • Proportioning valve, if equipped. See PROPORTIONING VALVE  under REMOVAL & INSTALLATION.
        • Brake Pressure Modulator Valve (BPMV). See appropriate ANTI-LOCK article.
    3. 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.
    4. Refill and bleed the hydraulic brake system. See MANUAL BLEEDING  or PRESSURE BLEEDING  under BLEEDING, FILLING & FLUSHING BRAKE SYSTEM.
    After repairs and/or replacements, go to step  9.
  6. Perform the following:
    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 procedure(s):
      • Master cylinder. See MASTER CYLINDER  under OVERHAUL or MASTER CYLINDER  under REMOVAL & INSTALLATION. Replace the brake master cylinder reservoir cap diaphragm.
      • Front brake hose. See FRONT BRAKE HOSE  under REMOVAL & INSTALLATION.
      • Rear brake hose. See REAR BRAKE HOSE  under REMOVAL & INSTALLATION.
      • Brake pipe. See BRAKE PIPE  under REMOVAL & INSTALLATION.
      • Front brake caliper. See BRAKE CALIPER  under REMOVAL & INSTALLATION. See BRAKE CALIPER  under OVERHAUL.
      • Wheel cylinders. See WHEEL CYLINDERS  under REMOVAL & INSTALLATION.
      • Proportioning valve, if equipped. See PROPORTIONING VALVE  under REMOVAL & INSTALLATION.
      • Brake Pressure Modulator Valve (BPMV). See appropriate ANTI-LOCK article.
    2. If you repaired or replaced any of the brake system components listed, bleed the hydraulic brake system. While bleeding the hydraulic brake system, observe for the following:
      • 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.
      See MANUAL BLEEDING  or PRESSURE BLEEDING  under BLEEDING, FILLING & FLUSHING BRAKE SYSTEM.
    If problem was found and corrected, go to next step. If problem was not found, go to step  12.
  7. If there is 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 is not air in the system at a bleeder valve location other than repair location, go to next step.
  8. If the flow of brake fluid is unrestricted and even per axle during the bleeding procedure, go to next step. If the flow of brake fluid is not unrestricted and even, go to step  10.
  9. Inspect the hydraulic function of the brake calipers/wheel cylinders for proper operation. See BRAKE COMPONENT OPERATION VISUAL INSPECTION  under ADJUSTMENTS & INSPECTIONS. If the hydraulic function of the brake calipers/wheel cylinders is operating properly, go to step  21. If the hydraulic function of the brake calipers/wheel cylinders is not operating properly, go to step  14.
  10. If the flow of brake fluid is restricted or uneven through front axle hydraulic components during the bleeding procedure, go to step  13. If the flow of brake fluid is not restricted or uneven, go to next step.
  11. If the flow of brake fluid is 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/wheel cylinders for proper operation. See BRAKE COMPONENT OPERATION VISUAL INSPECTION  under ADJUSTMENTS & INSPECTIONS. If the hydraulic function of the brake calipers/wheel cylinders is operating properly, go to step  15. If the hydraulic function of the brake calipers/wheel cylinders is not operating properly, 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.
    If the flow of brake fluid is unrestricted and the caliper piston moves freely, go to step  17. If the flow of brake fluid and caliper piston is not as specified, go to next step.
  14. Repair or replace any brake caliper/wheel cylinder that was not operating properly. Refer to the appropriate procedure(s). After 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 MANUAL BLEEDING  or PRESSURE BLEEDING  under BLEEDING, FILLING & FLUSHING BRAKE SYSTEM. If there is air in the system, go to step  19. If there is not air in the system, go to next step.
  16. If the flow of brake fluid is unrestricted and even per axle during the bleeding procedure, go to step  21. If the flow of brake fluid is not unrestricted and even, go to next step.
  17. Perform the following:
    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 & INSPECTIONS.
    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:
    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(s) at the restricted location.
    If problem was found and corrected, go to step  21. If problem was not found, go to next step.
  18. If equipped, replace the proportioning valve assembly. See PROPORTIONING VALVE  under REMOVAL & INSTALLATION. If equipped, replace the brake pressure modulator valve (BPMV), in order to correct the hydraulic brake dynamic rear proportioning mechanical operation. See appropriate ANTI-LOCK article. After 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. If problem was found and corrected, go to step  21. If problem was not found, go 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 OVERHAUL or MASTER CYLINDER  under REMOVAL & INSTALLATION. If problem was found and corrected, go to next step. If problem was not found, go to SYMPTOM INDEX  table.
  21. Install or connect components that were removed or disconnected during diagnosis. Install or connect components that were removed or disconnected during diagnosis. If procedure is completed, the hydraulic brake system is okay. Go to SYMPTOM INDEX  table.
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