1. Master Cylinder Issues
* Internal Leak: The most common culprit is a leak within the master cylinder itself. This can be caused by worn seals, a damaged piston, or even corrosion.
* Low Brake Fluid: Check the fluid level in the master cylinder reservoir. If it's low, you need to top it off and look for leaks.
* Air in the Lines: If air gets into the brake lines, it can cause a spongy or soft pedal. You'll need to bleed the brakes to remove the air.
2. Brake Booster Problems
* Vacuum Leak: The brake booster uses engine vacuum to assist braking. A leak in the booster's vacuum lines or the booster itself will reduce its effectiveness, leading to a soft pedal.
* Booster Failure: The booster diaphragm can tear or fail, preventing it from providing any assistance.
3. Other Possibilities
* Caliper Piston Sticking: Even after replacing the calipers, a piston could still stick, preventing proper brake pad contact.
* Worn Brake Lines: If the brake lines are old and worn, they might be expanding under pressure, leading to a soft pedal.
* Improper Bleeding: If the brakes weren't bled correctly after the repairs, air trapped in the lines could cause a soft pedal.
Troubleshooting Steps:
1. Check the Brake Fluid: Ensure the fluid level is full and clean. If it's low, top it off and look for leaks.
2. Check for Leaks: Examine the master cylinder, calipers, wheel cylinders, brake lines, and hoses for leaks.
3. Test the Booster: Start the engine and pump the brake pedal. If the pedal goes down easily and stays down when you hold it, the booster is likely working correctly. If it's hard to push and you feel a loss of assist, it could be a vacuum leak or booster failure.
4. Bleed the Brakes: If you suspect air in the lines, properly bleed the brakes. Start with the furthest wheel from the master cylinder and work your way towards it.
5. Inspect the Caliper Pistons: Make sure the pistons are moving freely in and out. If they are sticking, you may need to rebuild or replace the calipers.
6. Check the Brake Lines: Examine the lines for any bulges, swelling, or other signs of damage.
Important Note: If you suspect a problem with the master cylinder, booster, or brake lines, it's best to consult a qualified mechanic. These repairs can be complex and require specialized tools and knowledge.
