Why the Brakes Fail When the Engine is Running
* Power Brake Booster Malfunction: The power brake booster uses engine vacuum to assist your brake pedal. When the engine is running, the booster is working. If the booster has a leak, it can't build up enough pressure to give you braking force.
* Vacuum Leak in the System: A vacuum leak in the intake manifold or associated hoses could prevent the booster from functioning properly.
* Master Cylinder Failure: While you've just replaced it, there's a chance the new master cylinder could be faulty or incorrectly installed.
* Air in the System: Even though you bled the brakes, air can get trapped in the system, especially if you didn't bleed them thoroughly or if there's a leak somewhere.
Troubleshooting Steps
1. Check for Vacuum Leaks:
* Listen for hissing sounds: With the engine running, listen carefully for any hissing noises near the brake booster or intake manifold.
* Use a vacuum gauge: Connect a vacuum gauge to the vacuum line leading to the brake booster. The reading should be around 18-20 inches of mercury at idle. If it's significantly lower, you have a vacuum leak.
2. Inspect the Master Cylinder:
* Leaks: Check for fluid leaks around the master cylinder.
* Correct Installation: Ensure the master cylinder is installed correctly, including the pushrod adjustment.
* Reservoir Level: Make sure the brake fluid reservoir is full.
3. Re-Bleed the System:
* Thorough Bleeding: You may need to re-bleed the brake system several times to ensure all air is removed.
* Proper Bleeding Procedure: Follow the correct bleeding procedure for your vehicle (usually starting with the furthest wheel cylinder or caliper).
4. Check for Leaks in the Lines:
* Visual Inspection: Carefully inspect all brake lines for leaks.
* Pressure Test: If you suspect a leak, you can use a pressure tester to check for leaks in the lines.
Additional Tips
* Professional Diagnosis: If you're unable to find the problem, it's best to take your Caravan to a mechanic.
* Safety: Never drive your car with brakes that fail when the engine is running. It's extremely dangerous.
Important Safety Note: Always exercise caution when working on your brake system. If you're not comfortable with the repairs, it's best to take your vehicle to a qualified mechanic.
