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Hydraulic Brake System Diagnosis

WARNING: This page is about a different car, the 2023 GMC Sierra 3500 HD and 2023 GMC Sierra 2500 HD. However, it is still accessible from the selected car via links, so may be relevant.
Step Action Yes No
DEFINITION: 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. You will be directed by the appropriate Symptom table to go to other brake system diagnostic tables as appropriate.
1 Were you sent here from a Hydraulic Brake Symptom table? Go to Step 2  Go to Symptoms - Hydraulic Brakes 
2 Inspect and adjust the brake fluid level in the brake master cylinder. Brake Master Cylinder Reservoir Filling 
Was the brake fluid level low?		 Go to Step 5  Go to Step 3 
3
  1. Inspect the brake fluid for the following conditions, indicating brake fluid contamination:
    • Fluid separation, indicating two types of fluid are present
      • Swirled appearance - oil-based substance
      • Layered appearance - silicone-based substance
    • Fluid discoloration
      • Cloudy appearance - moisture
      • Dark appearance/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.
Do any of the above conditions exist?		 Go to Step 4  Go to Step 10 
4
  1. Flush the hydraulic brake system. Hydraulic Brake System Flushing 
  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 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.
  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. Hydraulic Brake System Bleeding 
Did you complete the operation and any required repairs and/or replacements?		 Go to Step 8  -
5
  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:
  2. If you repaired or replaced any of the brake system components listed, bleed the hydraulic brake system. Hydraulic Brake System Bleeding  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
Did you find and correct a condition?		 Go to Step 6  Go to Step 10 
6 Was there 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 17  Go to Step 7 
7 Was the flow of brake fluid unrestricted and even per axle during the bleeding procedure? Go to Step 8  Go to Step 9 
8 Inspect the hydraulic function of the brake calipers for proper operation. Hydraulic Brake Component Operation Visual Inspection 
Was the hydraulic function of the brake calipers operating properly?		 Go to Step 17  Go to Step 12 
9 Was the flow of brake fluid restricted or uneven through front axle hydraulic components during the bleeding procedure? Go to Step 13  Go to Step 10 
10 Inspect the hydraulic function of the brake calipers for proper operation. Hydraulic Brake Component Operation Visual Inspection 
Was the hydraulic function of the brake calipers operating properly?		 Go to Step 13  Go to Step 11 
11 Determine if the brake caliper is restricting the flow of brake fluid and/or not operating properly.
Brake Caliper Inspection
Was the flow of brake fluid unrestricted and did the caliper piston move freely?		 Go to Step 15  Go to Step 12 
12 Repair or replace any brake caliper that was not operating properly. Refer to the appropriate procedure:
Did you complete the repair and/or replacement?		 Go to Step 17  -
13 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. Hydraulic Brake System Bleeding 
Was there air in the system?		 Go to Step 17  Go to Step 14 
14 Was the flow of brake fluid unrestricted and even per axle during the bleeding procedure? Go to Step 17  Go to Step 15 
15
  1. Inspect the hydraulic brake pipes and flexible brake hoses for signs of a fluid restriction; such as being bent, kinked, pinched or damaged. Brake Pipe and Hose 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:
  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.
Did you find and correct a condition?		 Go to Step 17  Go to Step 16 
16
  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.
Did you find and correct a condition?		 Go to Step 17  Go to Step 3 
17 Install or connect components that were removed or disconnected during diagnosis.
Did you complete the operation?		 Hydraulic Brake System OK
Return to Symptom Table		 -
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