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Hydraulic Brake System Testing

  1. If you were sent here from SYMPTOMS - HYDRAULIC BRAKE  , go to next step. If you were not sent here from SYMPTOMS - HYDRAULIC BRAKE  , see DIAGNOSTIC STARTING POINT - HYDRAULIC BRAKES  under TROUBLE SHOOTING.
  2. Inspect and adjust the brake fluid level. See MASTER CYLINDER RESERVOIR FILLING  under BLEEDING 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 contamination:
    • A swirled appearance indicates an oil-based substance.
    • A layered appearance indicates a silicone-based substance.
    • A cloudy appearance indicates moisture.
    • A dark appearance or suspended particles in fluid indicate 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 contamination:
    • A swirled appearance indicates an oil-based substance.
    • A layered appearance indicates a silicone-based substance.
    • A cloudy appearance indicates moisture.
    • A dark appearance or suspended particles in fluid indicate 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 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:
    1. Remove ALL of the following components that utilize rubber seals or linings.
    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 containing rubber seals or linings.
      • Master Cylinder Overhaul or Master Cylinder Replacement; 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.
      • Brake hose replacement - front.
      • Brake hose replacement - rear.
      • Brake caliper overhaul or brake caliper replacement.
      • Wheel cylinder replacement.
      • Proportioning valve replacement.
      • Load sensing proportioning valve replacement.
      • Brake Pressure Modulator Valve (BPMV). See appropriate ANTI-LOCK article.
      See REMOVAL & INSTALLATION  or OVERHAUL  for procedures.
    5. If the brake fluid was not contaminated with an oil-based fluid, but was contaminated with water or dirt, 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  under BLEEDING BRAKE SYSTEM.
    6. Did you complete the operation and any required repairs and/or replacements? If so, 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):
    • Master Cylinder Overhaul Or Master Cylinder Replacement
    • Brake Hose Replacement - Front
    • Brake Hose Replacement - Rear
    • Brake Pipe Replacement
    • Brake Caliper Overhaul Or Brake Caliper Replacement
    • Wheel Cylinder Replacement
    • Proportioning Valve Replacement
    • Load Sensing Proportioning Valve Replacement
    • Brake Pressure Modulator Valve (BPMV). See appropriate ANTI-LOCK article.
    See REMOVAL & INSTALLATION  or OVERHAUL  . 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:
    • An unrestricted and even flow of brake fluid per axle.
    • The presence of air in the system at a bleeder screw location other than at the repair location, except if the brake master cylinder and/or the proportioning valve was replaced.
    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 screw location other than at the repair location, except if the brake master cylinder and/or the proportioning/combination valve 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 and the wheel cylinders for proper operation. See DISC BRAKE OPERATIONAL INSPECTION  under ADJUSTMENTS & INSPECTIONS. Was the hydraulic function of the brake calipers and the 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 and the wheel cylinders for proper operation. See DISC BRAKE OPERATIONAL INSPECTION  in DISC BRAKE SYSTEM under ADJUSTMENTS & INSPECTIONS. Was the hydraulic function of the brake calipers and the 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:
    • Raise and suitably support the vehicle. Remove the tire and wheel assemblies.
    • Open the suspected caliper bleeder screw.
    • 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 screw.
    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 procedures:
    • Brake Caliper Overhaul or Brake Caliper Replacement
    • Wheel Cylinder Replacement
    See REMOVAL & INSTALLATION  or OVERHAUL  for proper procedures. Did you complete the repair and/or replacement? If so, 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. See BLEEDING BRAKE SYSTEM  . Was there air in the system? If so, go to step  19 . If not, 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 front and rear brake hoses, and pipes, for signs of a fluid restriction; such as being bent, kinked or pinched. Replace any of the components listed found to be bent, kinked or pinched. If none of the components listed were bent, kinked or pinched, 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 proportioning valve. If the vehicle has ABS, see PROPORTIONING VALVE  under REMOVAL & INSTALLATION. If the vehicle does not have ABS, see LOAD SENSING PROPORTIONING VALVE  under REMOVAL & INSTALLATION. Did you complete the replacement? If so, 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 TEST  under SYSTEM TESTING. Repair or replace the brake master cylinder if it is found to be leaking brake fluid internally. See MASTER CYLINDER REMOVAL  or MASTER CYLINDER OVERHAUL  . Did you find and correct a condition? If so, go to next step.
  21. Install or connect components that were removed or disconnected during diagnosis. Did you complete the operation? If so, then system should be okay. If system is still not okay, go to step  2 .
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