* Air in the brake lines: This is the most common culprit. Even a tiny amount of air in the system can cause the brake warning light to stay on. You likely need to bleed the brakes thoroughly at each wheel. Failure to bleed the brakes correctly is a frequent mistake after master cylinder replacement. Make sure you follow the correct bleeding sequence for your Aerostar (consult your repair manual).
* Low brake fluid: Check the brake fluid level in the reservoir. If it's low, you may have a leak somewhere in the system (besides the master cylinder, which you've already replaced). Top it off, but be prepared to bleed the brakes again if necessary.
* Brake fluid level sensor: The reservoir has a sensor that triggers the warning light. The sensor itself might be faulty, or it might not be making proper contact. Check the sensor and its connection.
* Brake light switch: This switch is activated by the brake pedal and turns on the brake lights. A faulty switch can cause the brake warning light to remain on even if there’s nothing wrong with your brake system.
* ABS sensor or module: If your Aerostar has anti-lock brakes (ABS), a problem with the ABS sensor or module could trigger the warning light, even if unrelated to the master cylinder replacement.
* Wiring issue: There might be a short circuit or broken wire in the brake system's wiring harness, which affects the warning light circuit. Inspect the wiring for damage.
What to do:
1. Check and top off the brake fluid: This is the easiest first step.
2. Bleed the brakes: This is crucial and often overlooked. Follow the proper bleeding procedure for your vehicle.
3. Check the brake light switch: Ensure it's functioning correctly.
4. Inspect for leaks: Carefully look for any brake fluid leaks around the master cylinder, calipers, wheel cylinders, and brake lines.
5. Consult a repair manual: A repair manual specific to your 1996 Ford Aerostar will provide detailed diagrams and instructions for bleeding the brakes and troubleshooting brake system issues.
6. Have a professional diagnose the problem: If you're uncomfortable working on your brakes yourself, take your Aerostar to a qualified mechanic for diagnosis and repair. Brake system issues should not be taken lightly.
Ignoring brake system warning lights is extremely dangerous. Address this issue immediately.
