* Vacuum Leak in the Booster System: This is the most probable cause. The brake booster uses engine vacuum to assist pedal pressure. A leak in the booster itself, the vacuum lines connecting it to the intake manifold, or even a poorly sealing intake manifold gasket will dramatically reduce the booster's assistance, leading to a soft pedal when the engine is running. Check all vacuum lines for cracks, splits, or loose connections. Inspect the booster diaphragm for damage or leaks. Listen carefully around the intake manifold and booster for hissing sounds while the engine is running.
* Master Cylinder Internal Leak: While less likely given the good pedal feel with the engine off, a failing master cylinder could allow fluid to bypass the piston when under pressure from the booster. This leak would worsen with engine-running conditions, due to the increased pressure from the vacuum assist. Inspect the master cylinder for any signs of leakage.
* Faulty Brake Booster: The booster itself could be internally damaged, preventing it from providing sufficient assistance. This often involves a ruptured diaphragm inside the unit. Replacing the booster is usually necessary if it's faulty.
* Proportioning Valve (if equipped): While less common to cause this specific symptom, a sticking or malfunctioning proportioning valve could improperly distribute brake pressure, making the pedal feel spongy or fade under pressure.
Troubleshooting Steps:
1. Inspect Vacuum Lines: Start here! Visually inspect all vacuum lines related to the brake booster. Replace any damaged or questionable lines.
2. Listen for Leaks: With the engine running, carefully listen around the booster and intake manifold for any hissing sounds indicating a vacuum leak.
3. Check the Booster Diaphragm (Advanced): This requires removing the booster, which is more involved but might be necessary if you suspect a leak in the booster itself. A visual inspection is needed to determine the condition of the diaphragm.
4. Master Cylinder Inspection: Look for any signs of brake fluid leakage around the master cylinder.
5. Bleed the Brakes Again: After addressing potential vacuum leaks, bleed the brakes thoroughly again to remove any air that may have entered the system during the repair process. Bleeding should be done *with the engine running* this time to properly simulate the vacuum assist pressure.
6. Proportioning Valve Check (if applicable): If the problem persists and your Corvette has a proportioning valve, investigate its operation.
Important Safety Note: Always exercise caution when working on your brake system. Brake failure can be extremely dangerous. If you are not comfortable performing these repairs yourself, take your vehicle to a qualified mechanic. A properly functioning brake system is essential for safe driving.
