Owner's Problem Description
The owner reports that the brakes on their 1995 Buick Riviera are almost non-existent, pushing the pedal to the floor without stopping the vehicle effectively. An audible hissing sound is heard whenever the brakes are applied, regardless of speed. They suspect the brake booster may be the culprit and have checked the brake fluid, which appears to be at normal levels with no visible leaks.
Professional Technical Analysis
This 1995 Buick Riviera, equipped with an anti-lock braking system (ABS), is experiencing a significant braking issue characterized by a soft brake pedal that goes to the floor without engaging the brakes effectively. The audible hissing sound when applying the brakes suggests a potential vacuum leak, likely from the brake booster. Given the vehicle's age and typical wear patterns, a thorough inspection of the brake system is essential. A common failure in vehicles of this age includes the degradation of the brake booster diaphragm, which can lead to loss of power assist and cause the hissing noise. The brake fluid levels are adequate, indicating that the master cylinder and brake lines are not leaking. However, it is critical to inspect the master cylinder for internal leaks and the brake pads/shoes for wear. If the pads are worn to the metal, this can also cause significant braking issues. Furthermore, a malfunctioning brake booster can lead to inadequate brake performance due to insufficient vacuum assist. The system's interconnections must be carefully evaluated, including the relationship between the brake booster and the master cylinder pressure. The ABS system may also influence the overall braking efficiency, requiring diagnostic verification of its function and any fault codes that may be present.
Possible Causes
Most common causes (ordered by frequency):
- Brake Booster Failure: The brake booster diaphragm may have ruptured, causing a vacuum leak that results in the hissing noise and loss of power assist. This is common in older vehicles, particularly those exposed to extreme temperatures or moisture. The brake booster should provide a steady vacuum signal to the master cylinder, typically around 18-20 inches of mercury. If the diaphragm is compromised, vacuum levels will drop, leading to ineffective braking. - Audible hissing sound when brakes are applied, soft brake pedal with no resistance, vacuum gauge readings below 18 inches of mercury.
- Master Cylinder Failure: The master cylinder may have internal leaks, preventing adequate pressure from being built up in the brake lines. This could explain the soft pedal feel. Checking for fluid leakage around the seals is crucial. If the master cylinder is compromised, it may need to be replaced. Standard replacement cost is around $150-300. - Fluid check shows no external leaks; however, pedal feels soft and sinks to the floor without resistance, indicating a possible internal failure.
- Brake Pad Wear: If the brake pads are significantly worn down, they may not provide adequate friction, leading to poor braking performance. This would be accompanied by a soft pedal feel. Inspecting the thickness of the pads and checking for metal-to-metal contact is essential. Replacement cost for pads typically ranges between $100-200. - Visual inspection reveals thin brake pads, potential metal-on-metal contact observed, which could cause further damage if not addressed.
Diagnostic Steps
Professional Diagnosis Process
Follow these systematic steps to accurately diagnose the issue. Each step builds on the previous one to ensure accurate diagnosis.
- STEP 1 - Visual Inspection: Start with a thorough visual inspection of the brake system components, including the brake booster, master cylinder, and brake pads. Look for any signs of fluid leaks, damage, or wear. A careful check of the brake booster for cracks or deterioration is essential, especially around the vacuum connections. Use a flashlight to examine hard-to-see areas.
- STEP 2 - Vacuum Testing: Utilize a vacuum gauge to test the brake booster. It should maintain a vacuum level of 18-20 inches of mercury when the engine is running. If the vacuum is low, this indicates a potential diaphragm issue. Additionally, listen for any hissing sounds during testing to confirm leaks.
- STEP 3 - Master Cylinder Inspection: Remove the master cylinder from the brake booster and inspect for any signs of leakage, especially on the back side. If there’s evidence of fluid around the seals, it indicates a failure. This step is crucial as it can affect overall braking performance.
- STEP 4 - Brake Pad Thickness Check: Use a caliper to measure the brake pad thickness. Pads should have a minimum thickness of 3 mm to ensure effective braking. If the pads are below this threshold, they should be replaced immediately to avoid further damage.
