* Brake Booster Vacuum Leak: This is the most probable cause. The brake booster uses engine vacuum to assist in braking. A leak in the booster itself, its vacuum lines (often brittle and cracked in older cars), or even a leak in the intake manifold can reduce or eliminate the vacuum assist. When you brake hard, the extra load on the engine, combined with the loss of vacuum assist, can cause the engine to stall.
* Dirty or Failing Brake Booster: Even without a leak, a worn-out or dirty brake booster might not be providing sufficient assist, leading to stalling under heavy braking.
* Low Engine Idle Speed: If your engine's idle speed is already low, the extra load of braking can push it below the point where the engine can maintain itself, causing a stall. This can be due to a variety of issues like a dirty throttle body, a failing idle air control (IAC) valve, or a vacuum leak elsewhere in the intake system.
* Transmission Problems: Less likely, but possible, especially if the stalling only happens under certain braking conditions (e.g., downshifting). A problem with the transmission's torque converter or linkage could put extra strain on the engine when braking.
* Ignition System Issues: While less common in relation to braking, a weak ignition system could cause the engine to falter under the added load of braking. This is less likely than the vacuum issues.
Troubleshooting Steps (in order of likelihood):
1. Check for Vacuum Leaks: Visually inspect all vacuum lines connected to the brake booster and intake manifold. Look for cracks, loose connections, or collapsed lines. You can also use a vacuum gauge to test the vacuum at the booster while someone applies the brakes. Low or fluctuating vacuum indicates a leak.
2. Inspect the Brake Booster: While this is harder to diagnose visually, a failing booster often makes groaning noises or feels spongy when the brakes are applied.
3. Check Idle Speed: With the engine running, observe the idle speed. If it's unusually low (should be around 650-800 RPM, check your owner's manual), then a further investigation into the idle system components is needed.
4. Consider the Transmission (if only under specific conditions): If the problem only occurs under specific braking conditions, such as downshifting, then transmission issues are a possibility. This requires a more specialized diagnostic.
It's best to have a qualified mechanic diagnose and repair this issue. Driving with a faulty brake booster is dangerous because it reduces braking power.
