Owner's Problem Description
The owner reports that the brake pedal on their 1988 Nissan 2WD truck goes all the way to the floor despite having bench bled the master cylinder and bled the wheel cylinders in the correct order. They have replaced the master cylinder three times and checked for external leaks without success. The owner is experienced with brake systems and is unsure if they are missing something in the diagnosis.
Professional Technical Analysis
The 1988 Nissan 2WD truck is experiencing a critical brake issue where the pedal goes to the floor, indicating a potential hydraulic failure or air in the system. The vehicle is equipped with a standard hydraulic brake system, and the owner has already replaced the master cylinder three times, indicating a persistent issue likely not related to the master cylinder itself. The absence of external leaks suggests that the problem lies within the hydraulic system. The vehicle's load sensing valve, which is typically present in many vehicles, appears to be missing according to the owner's investigation, raising concerns about the braking system's integrity. It is crucial to ensure that the pushrod from the brake pedal to the master cylinder is correctly adjusted and functioning properly, as a misalignment can lead to inadequate pressure generation, causing the pedal to drop. Additionally, the rubber brake lines must be inspected for expansion under pressure, which could indicate they are failing internally. A thorough inspection and bleeding of the proportioning valve might be warranted, as neglecting this component can lead to improper brake fluid distribution, exacerbating the issue. Given the owner's experience, a systematic approach to re-evaluating the entire brake system, including checking all connections, seals, and possible hidden leaks, should be employed to identify the root cause of this persistent brake failure.
Possible Causes
Most common causes (ordered by frequency):
- Master Cylinder Pushrod Misalignment: If the pushrod from the brake pedal to the master cylinder is not properly adjusted, it can prevent the master cylinder from generating sufficient hydraulic pressure. The pushrod should have no play and should be correctly aligned to ensure effective brake operation. Testing the pushrod adjustment by measuring the distance from the pedal to the master cylinder can ensure proper engagement. - Brake pedal feels loose, visible gap between pedal and pushrod, difficulty in achieving brake pressure.
- Rubber Brake Line Expansion: The rubber brake lines may be expanding under pressure, leading to a loss of effective braking force. This can occur due to old or degraded hoses that cannot withstand the pressure generated by the brake system. Testing the brake lines for expansion by applying the brake and observing for any swelling or softening is essential. - Soft brake pedal feel, visual inspection of brake lines shows signs of wear or damage.
- Proportioning Valve Malfunction: If the proportioning valve is faulty or not bled properly, it can cause uneven pressure distribution to the brake system, leading to a soft pedal or total loss of braking force. This valve must be bled to remove any trapped air, which is often overlooked in brake system repairs. Testing the valve can involve applying pressure and checking for fluid flow discrepancies. - Inconsistent brake response, visible air bubbles in brake fluid during bleeding.
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 - Brake System Inspection: Begin with a thorough visual inspection of the entire brake system, including the condition of all brake lines, calipers, and connections. Pay close attention to the rubber lines for any signs of bulging or wear. Verify the pushrod adjustment from the brake pedal to the master cylinder, ensuring there is no play and it is correctly aligned.
- STEP 2 - Brake Line Testing: Conduct a pressure test on the rubber brake lines. Apply the brake pedal and observe if any lines expand or show signs of leaking under pressure. This can help identify if the lines are the source of the failure. If expansion occurs, replace the hoses with high-quality OEM replacements.
- STEP 3 - Proportioning Valve Bleeding: If the truck is equipped with a proportioning valve, it is critical to bleed it as part of the system. Locate the valve and follow the proper bleeding procedure, which typically involves applying the brake while releasing air from the valve. Use a pressure bleeder if available to simplify the process.
- STEP 4 - Test Drive: After completing the above steps, perform a test drive to evaluate brake performance. Pay attention to pedal feel and response during braking. If the issue persists, further investigation into the master cylinder and other hydraulic components may be necessary.
