* Proportioning Valve Issues: These valves regulate brake pressure between the front and rear axles. A malfunctioning or incorrectly adjusted proportioning valve could cause fade, especially under heavy braking. It might not be immediately apparent if the valve itself is faulty, or if its adjustment is incorrect for the specific vehicle and load.
* Power Booster Problems: While you replaced the master cylinder, the power booster itself could be failing. A leaking or malfunctioning booster won't provide adequate assistance, leading to a soft pedal and fade. A vacuum leak in the booster system could also be a culprit. Check the vacuum line connections to the booster and ensure it holds vacuum properly.
* Wheel Cylinders/Caliper Issues (Internal): Even with new wheel cylinders or calipers, internal problems might exist. A slightly scored piston or seal could cause reduced braking efficiency, especially as the system heats up under heavy braking (leading to fade). It’s possible a microscopic defect wasn't noticed during initial inspection.
* Improper Bleeding: Though you say the system was bled, air can still remain trapped in hard-to-reach spots in long brake lines or within the ABS unit (if equipped). A thorough and meticulous bleeding process is crucial, sometimes requiring specialized tools or techniques. Consider bleeding the system from furthest point (rear brakes) to closest (master cylinder), and using a pressure bleeder to assist.
* Master Cylinder Internal Problem: While you got a new master cylinder, there is always a small possibility that it is faulty. Some internal components can fail without immediately obvious symptoms.
* Brake Lines/Hoses: Internal corrosion or restrictions in the brake lines or hoses, even new ones, can cause a pressure drop and brake fade. The lines may appear fine on the outside but have internal issues.
* Overheating: The brake system itself (calipers, rotors, drums) might overheat under heavy use. This is less likely with new components, but extremely heavy loads or extended downhill braking could still lead to fade.
Troubleshooting Steps:
1. Check for Vacuum Leaks: Test the vacuum system related to the power booster.
2. Inspect the Proportioning Valve: Verify its operation and adjust it according to specifications (if adjustable).
3. Re-bleed the System: Perform a thorough and meticulous bleeding procedure, possibly with a pressure bleeder.
4. Inspect Wheel Cylinders/Calipers: Carefully examine them internally for any signs of damage or leaks.
5. Inspect Brake Lines and Hoses: Look for any kinks, restrictions, or evidence of internal corrosion.
6. Test the Master Cylinder (if possible): If you have access to a brake bench tester, this can confirm its functionality.
7. Check for Overheating: Inspect brake components (rotors, drums, calipers) for excessive heat after heavy braking.
If you've checked all these and still have fade, it's time to seek professional help from a qualified brake mechanic. They have the tools and expertise to diagnose more subtle issues.
