Here's a breakdown of the likely culprits:
* Brake Booster Vacuum Leak: The brake booster uses engine vacuum to assist braking. A leak in the booster's vacuum lines or a faulty booster itself can cause a sudden loss of vacuum when the brakes are applied. This loss of vacuum can cause a momentary drop in engine speed or even a stall, especially at idle or low RPMs. This is the most common reason.
* Faulty Brake Switch: The brake light switch is a safety device that usually cuts off the power to the engine in case of an emergency stop. A faulty switch can send the wrong signal to the engine control module (ECM), causing the engine to shut down. However, this is less likely to cause the engine to cut out smoothly. It's more likely to cause erratic behavior.
* Transmission Issues (Automatic Transmission): In automatic transmission vehicles, a problem within the transmission itself, like a low fluid level or internal failure, could cause the engine to cut out under braking stress.
* Electrical Problems: While less common, a short circuit or other electrical malfunction affecting the engine's power supply could also cause a stall when applying the brakes. This would generally be accompanied by other electrical issues in the vehicle.
* Fuel System Problems: Although rare in relation to braking, severe fuel starvation (e.g., a clogged fuel filter) could cause the engine to sputter or stall, particularly under high load from braking and the vacuum assist.
* Engine Vacuum Issues (Beyond Brake Booster): A general loss of engine vacuum due to leaks elsewhere in the intake system could also contribute, though it would likely manifest in other ways as well (rough idle, poor performance).
Troubleshooting:
If your engine is cutting off when you brake, it's crucial to have a qualified mechanic diagnose the problem. Attempting to diagnose and fix this yourself can be dangerous. A proper diagnosis involves checking the brake booster vacuum, inspecting the brake light switch, and possibly testing the engine's vacuum and other systems.
