GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Saturn 🢒 2004 🢒 L300/LW300 🢒 Brakes 🢒 Hydraulic Brakes 🢒 Hydraulic Brake System Diagnosis

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 Brake Symptom table?

Go to Step 2

Go to Diagnostic Starting Point - Hydraulic Brakes

2

Inspect and adjust the brake fluid level in the brake master cylinder. Refer to Master Cylinder Reservoir Filling .

Was the brake fluid level low?

Go to Step 3

Go to Step 4

3
  1. Inspect the brake fluid for the following conditions, indicating brake fluid contamination:
    2. • 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 5

Go to Step 6

4
  1. Inspect the brake fluid for the following conditions, indicating brake fluid contamination:
    2. • 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 5

Go to Step 12

5
  1. Flush the hydraulic brake system. Refer to 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.
    3. 2.1. Remove ALL of the following components listed. Each component contains internal rubber seals/linings which have been contaminated.
    2.2. Clean out the hydraulic brake pipes using denatured alcohol, or equivalent.
    2.3. Dry the brake pipes using non-lubricated, filtered air.
    2.4. Repair or replace ALL of the following components listed. Each component contains internal rubber seals/linings which have been contaminated.
       • Brake master cylinder reservoir: 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 Pressure Modulator Valve Replacement in Antilock Brake System
  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. Refer to Hydraulic Brake System Bleeding .

Did you complete the operation and any required repairs and/or replacements?

Go to Step 9

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6
  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. • Brake master cylinder reservoir cap diaphragm
    •  Brake Pressure Modulator Valve Replacement in Antilock Brake System
  2. If you repaired or replaced any of the brake system components listed, bleed the hydraulic brake system. Refer to Hydraulic Brake System Bleeding . While bleeding the hydraulic brake system, observe for the following conditions:
    3. • 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 7

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 was replaced?

Go to Step 19

Go to Step 8

8

Was the flow of brake fluid unrestricted and even per axle during the bleeding procedure?

Go to Step 9

Go to Step 10

9

Inspect the hydraulic function of the brake calipers for proper operation. Refer to Hydraulic Brake Component Operation Visual Inspection .

Was the hydraulic function of the brake calipers/wheel cylinders operating properly?

Go to Step 21

Go to Step 14

10

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 11

11

Was the flow of brake fluid restricted or uneven through rear axle hydraulic components during the bleeding procedure?

Go to Step 17

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12

Inspect the hydraulic function of the brake calipers for proper operation. Refer to Hydraulic Brake Component Operation Visual Inspection .

Was the hydraulic function of the brake calipers operating properly?

Go to Step 15

Go to Step 13

13

Determine if the brake caliper is restricting the flow of brake fluid and/or not operating properly:

  1. Raise and support the vehicle. Refer to Lifting and Jacking the Vehicle in General Information.
  2. Remove the tire and wheel assemblies. Refer to Tire and Wheel Removal and Installation in Tires and Wheels.
  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?

Go to Step 17

Go to Step 14

14

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 21

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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. Refer to Hydraulic Brake System Bleeding .

Was there air in the system?

Go to Step 19

Go to Step 16

16

Was the flow of brake fluid unrestricted and even per axle during the bleeding procedure?

Go to Step 21

Go to Step 17

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. Refer to 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 21

Go to Step 18

18

Replace the brake pressure modulator valve (BPMV), in order to correct the hydraulic brake dynamic rear proportioning mechanical operation. Refer to Brake Pressure Modulator Valve Replacement in Antilock Brake System.

Did you complete the replacement?

Go to Step 21

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19
  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 21

Go to Step 20

20
  1. Inspect the brake master cylinder for internal fluid leaks. Refer to Brake System Internal Leak Test .
  2. Repair or replace the brake master cylinder if it is found to be leaking brake fluid internally. Refer to Master Cylinder Overhaul or Master Cylinder Replacement .

Did you find and correct a condition?

Go to Step 21

Return to Symptom Table

21

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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