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Brake Assist System Diagnosis

WARNING: This page is about a different car, the 2003 GMC Sonoma, 2003 GMC Jimmy, 2003 Chevrolet S10 Pickup, and 2003 Chevrolet Blazer. However, it is still accessible from the selected car via links, so may be relevant.
  1. Were you sent here from SYMPTOMS - HYDRAULIC BRAKES  under TROUBLE SHOOTING? If so, go to the next step. If not, see STARTING POINT - HYDRAULIC BRAKES .
  2. Inspect for proper brake pedal travel. See BRAKE PEDAL TRAVEL MEASUREMENT AND INSPECTION  under ADJUSTMENTS & INSPECTION. Is the brake pedal travel distance within the acceptable limits? If so, go to step  5. If not, go to the next step.
  3. Inspect for worn, missing, misaligned, bent or damaged brake pedal system components. For the brake pedal pushrod component inspection, see BRAKE PEDAL PUSHROD INSPECTION  under ADJUSTMENTS & INSPECTION. Inspect the brake pedal bushings for excessive wear and/or damage and inspect the brake pedal for a misaligned, bent and/or damaged condition. Replace the brake pedal system components that are worn, missing, misaligned, bent or damaged, See BRAKE PEDAL  and/or VACUUM BRAKE BOOSTER  under REMOVAL & INSTALLATION. Did you find and replace any worn, missing, misaligned, bent or damaged brake pedal system components? If so, go to the next step. If not, go to step  5.
  4. Reinspect for proper brake pedal travel. See BRAKE PEDAL TRAVEL MEASUREMENT AND INSPECTION  under ADJUSTMENTS & INSPECTION. Is the brake pedal travel distance within the acceptable limits? If so, go to step  8. If not, go to the next step.
  5. Check the engine vacuum source that supplies vacuum to the vacuum brake booster. See BRAKE SYSTEM VACUUM SOURCE TEST  under SYSTEM TESTS. Is the vacuum reading within the acceptable limits? If so, go to the next step. If not, diagnose low vacuum condition. See appropriate ENGINE article in ENGINES.
  6. During the vacuum source inspection, did the vacuum booster check valve operate properly? If so, go to step  8. If not, go to the next step.
  7. Replace the vacuum booster check valve. See VACUUM BRAKE BOOSTER CHECK VALVE AND/OR HOSE  under REMOVAL & INSTALLATION. After repairs are complete, go to the next step.
  8. Pump the brake pedal several times until the brake pedal becomes hard. Maintain moderate foot pressure on the brake pedal and start the engine. Observe pedal operation. Did the brake pedal drop slightly, then remain firm after the engine was started? If so, go to the next step. If not, go to step  12.
  9. Release the brake pedal. Turn ignition off, then wait 15 seconds. Pump the brake pedal 2 times to check for vacuum booster available vacuum reserve. Observe pedal effort. If operating properly, the vacuum brake booster should maintain assist for at least 2 pedal applications. Did the brake pedal effort increase significantly? If so, go to step  12. If not, go to the next step.
  10. Inspect the brake pedal pushrod and the brake pedal for misalignment, a bent condition and/or damage. Is the brake pedal pushrod and/or the brake pedal misaligned, bent or damaged? If so, go to the next step. If not, go to step  13.
  11. Replace the brake pedal pushrod and/or the brake pedal, as necessary. See BRAKE PEDAL  and/or VACUUM BRAKE BOOSTER  under REMOVAL & INSTALLATION. When repair is complete go to step  13.
  12. Replace the vacuum brake booster. See VACUUM BRAKE BOOSTER  under REMOVAL & INSTALLATION. When repair is complete go to the next step.
  13. Install or connect any components that were removed or disconnected during diagnosis. Did you complete the operation? If so, the Brake Assist System is okay. If not, return to SYMPTOMS - HYDRAULIC BRAKES  under TROUBLE SHOOTING.
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