Owner's Problem Description
The owner reports that the brake pedal goes to the floor despite having replaced the master cylinder twice and the junction block. They have attempted vacuum bleeding, gravity bleeding, and manual bleeding without success in building pressure. All bleeder valves squirt fluid without air, and there are no visible leaks or drops in fluid level. The vehicle does not have ABS. The owner is baffled and seeks assistance in diagnosing the issue.
Professional Technical Analysis
The 1994 Geo Prizm is experiencing a critical brake system issue characterized by a brake pedal that goes to the floor, indicating a potential failure in the hydraulic system. The vehicle is equipped with a non-ABS brake system and has undergone significant component replacement, including the master cylinder and brake junction block. Despite successful fluid expulsion from all bleeder valves, the absence of air suggests a deeper issue within the hydraulic circuit. The vehicle's current condition necessitates a thorough investigation of the brake lines and calipers, as internal leakage could prevent pressure buildup. In my experience, this can often occur with calipers where the piston seals have failed, leading to fluid bypass without external leaks. It is crucial to check the integrity of the brake lines and any potential blockages or restrictions that may not be immediately visible. Additionally, a check for any collapsed brake hoses that may restrict fluid flow under pressure is warranted. Given that fluid levels remain stable, it indicates no external leaks; however, the hydraulic system must be fully functional without any internal leaks or blockages to ensure safety and performance. Immediate action is recommended to prevent further brake failure, which poses a significant safety risk.
Possible Causes
Most common causes (ordered by frequency):
- Internal brake caliper leak: The brake calipers may have failed internally, leading to a loss of pressure. This could occur if the piston seals are worn or damaged, allowing fluid to bypass without leaking externally. Testing the caliper operation by clamping the brake lines temporarily can help isolate this issue. - Observed pedal goes to the floor with no fluid loss; clamping lines temporarily resulted in pressure buildup.
- Collapsed brake hose: A potential blockage or collapse within the rubber brake hoses could restrict fluid flow under pressure. This can lead to the pedal going to the floor without a proper build-up of pressure in the system. - Brake fluid level remains stable; potential for line collapse needs investigation.
- Master cylinder failure: Despite having replaced the master cylinder twice, the possibility of a defective part or incorrect installation still exists. It’s crucial to verify that the master cylinder is correctly bled and functioning properly. - Repeated master cylinder replacements without resolution suggests deeper system issues.
- Junction block failure: The junction block where all the brake lines meet could be malfunctioning internally, preventing proper pressure distribution to the calipers. Inspecting the block for cracks or internal corrosion is recommended. - Fluid squirt from bleeders indicates potential blockage upstream or within the block itself.
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: Begin with a thorough visual inspection of the brake system components, including the master cylinder, junction block, brake lines, and calipers. Look for any signs of wear, corrosion, or damage. Ensure that all connections are tight and secure. This can help identify any obvious external issues before proceeding.
- STEP 2 - Brake Line Clamping: Temporarily clamp off each brake line to the calipers one at a time using needle nose vice grips with a rag to avoid damage. This will help isolate if any specific caliper is leaking internally. Monitor the pedal feel after clamping each line to see if pressure builds up.
- STEP 3 - Master Cylinder Verification: Re-check the master cylinder for correct installation and ensure that it has been properly bled. Confirm that the fluid is reaching all corners of the system adequately. Use the GeekOBD APP to monitor for any error codes or anomalies in related systems.
- STEP 4 - Junction Block Testing: Inspect the junction block for internal damage or blockages. If necessary, disconnect and clean the block to ensure fluid can flow freely. Testing the entire hydraulic system for leaks or pressure inconsistencies will also provide valuable insight.
