* Improper Bleed: The most common culprit. Air trapped in the system, even a tiny bubble, can interfere with piston retraction. A thorough and proper bleed procedure is crucial, potentially requiring multiple bleeds and different techniques (pressure bleeding, vacuum bleeding). Insufficient bleeding leads to uneven pressure distribution and potentially sticking calipers.
* Caliper Piston Issues: Despite being new, calipers might have manufacturing defects or internal problems preventing smooth piston retraction. A sticking caliper piston can be caused by:
* Seized piston: Corrosion or debris could cause the piston to bind in its bore.
* Dust boot damage: Damaged dust boots allow contaminants to enter the caliper, leading to seizing.
* Caliper bore damage: Scratches or imperfections in the caliper bore can cause the piston to bind.
* Brake Line Routing and Restriction: Although less likely after a full replacement, a kinked or improperly routed brake line can restrict fluid flow and cause pressure imbalances, leading to one caliper sticking more than the others.
* Master Cylinder Issues: While you replaced it, there's a small chance of a manufacturing defect in the new master cylinder itself, causing uneven pressure distribution.
* Brake Pad Issues: While less likely to cause *sticking* per se, improperly seated brake pads or pads that are too thick/oversized could create excessive pressure and *feel* like sticking.
* Fluid Contamination: Even with new fluid, contamination could have occurred during the replacement process. Dirt or moisture can easily cause issues.
Troubleshooting Steps:
1. Re-bleed the brakes: Use a pressure bleeder if possible for the most effective bleed. Pay close attention to the order and technique.
2. Inspect the calipers: Carefully examine the calipers for any signs of damage, sticking pistons, or damaged dust boots. Try to manually retract the pistons (with appropriate caution and tools) to see if they move freely.
3. Check brake lines: Ensure the lines are routed correctly and free of kinks or restrictions.
4. Inspect brake pads: Make sure they're seated correctly and are not oversized.
5. Check master cylinder operation: Ensure the master cylinder is functioning correctly and that fluid pressure is distributed evenly.
6. Consider professional help: If you've exhausted these steps and the problem persists, it's best to consult a qualified mechanic. They have specialized tools and expertise to diagnose the problem accurately.
Sticking brakes are a safety hazard. Do not drive the vehicle until the issue is resolved.
