* Brake Booster Vacuum Leak: This is the most common cause. The brake booster uses engine vacuum to assist braking. A leak in the booster itself, its vacuum lines, or the intake manifold can cause a significant drop in vacuum when the brakes are applied. This loss of vacuum can starve the engine of the air it needs to run smoothly, leading to stalling.
* Dirty or Failing Throttle Position Sensor (TPS): The TPS tells the engine control module (ECM) the position of the throttle. If it's dirty or malfunctioning, applying the brakes (which often slightly reduces engine speed) could cause the TPS to send incorrect signals to the ECM, resulting in a sudden fuel cutoff or improper air/fuel mixture, causing a stall.
* Failing Idle Air Control (IAC) Valve: The IAC valve controls the idle speed. When the brakes are applied, there's a momentary drop in engine speed. A failing IAC valve may not be able to compensate for this drop, leading to the engine stalling.
* Alternator Problems: While less likely to be the *direct* cause of stalling upon braking, a failing alternator could be a contributing factor. If the alternator isn't charging the battery properly, applying the brakes (which puts a significant load on the electrical system) could cause a voltage drop that's enough to interrupt engine operation.
* Faulty Brake Light Switch: Though less common, a short circuit or failure in the brake light switch could draw excessive current, causing a voltage drop similar to an alternator problem and resulting in a stall.
* Vacuum leaks in the engine: Leaks elsewhere in the intake manifold system can also lead to vacuum loss causing this problem, though it would likely affect the engine more generally.
Troubleshooting Steps (in order of likelihood):
1. Check vacuum lines: Inspect all vacuum lines leading to and from the brake booster for cracks, leaks, or loose connections. A visual inspection might reveal a problem; a shop vacuum can be used to test for leaks.
2. Inspect the brake booster: Check the brake booster for any visible signs of damage or leaks. This often requires a mechanic's expertise.
3. Check the TPS and IAC valve: These components can be tested with a multimeter, or by professional diagnostic scanning tools. Cleaning the TPS may resolve the issue if it's dirty.
4. Test the alternator: Have the alternator tested for proper output voltage.
5. Check the brake light switch: This is easier to replace than to test. Often a relatively inexpensive part.
Important Note: Diagnosing this issue reliably requires some mechanical knowledge or access to diagnostic equipment. If you're not comfortable working on your car, taking it to a qualified mechanic is the safest and most efficient way to resolve the problem. Ignoring the issue could lead to dangerous braking conditions.
