Why Is There No Brake When the Car Stalls?
3 Answers
The reason there is no brake when the car stalls is that the braking system is not directly driven by the pedal but relies on a brake vacuum booster to assist braking. The booster pump is only driven when the engine is running. Therefore, it is normal for the brake to be unavailable after the engine stalls. Principle of Braking: The car's brake pedal is connected to the vacuum booster. When the brake pedal is pressed, the vacuum booster comes into play, and the pushrod forcefully connects to the master brake cylinder. The master cylinder then transmits hydraulic fluid to the brake calipers through hydraulic lines. At this point, the piston presses the brake pads, completing the braking operation. Other Causes of Brake Failure: Tight or non-retracting parking brake cables: A non-retracting parking brake cable can easily cause dragging brakes, leading directly to overheating of the brake discs or abnormal brake noises. Air in the master brake cylinder: When changing brake fluid, bleeding is crucial. If air remains in the brake fluid, it can severely affect braking performance and even cause brake failure. Excessive water content in brake fluid: This can be detected using a brake fluid tester. Once the water content exceeds the standard, it indicates deterioration of the brake fluid. Brake fluid also requires regular maintenance and replacement, especially when water content is too high.
Once when I was driving home from work, the engine suddenly stalled. When I pressed the brake pedal, I found it extremely stiff, and the car almost rolled away, which scared me. After consulting a mechanic friend, I learned that modern cars use vacuum boosters to make braking easier, relying on the engine's operation to create vacuum pressure. When the engine stalls, the vacuum quickly dissipates, and the power assist disappears. You have to manually exert all your strength to press the pedal to slow down, which is ten times more effort than usual. However, the mechanical components of the brakes themselves are still functional, and if you apply enough force, you can still stop the car. In such a situation, you should quickly turn the key to restart the engine. If the engine stalls on the highway, it's even more dangerous—first, pull over safely with hazard lights on and check the car. Regular maintenance should include checking the vacuum lines for leaks and promptly repairing any engine issues to prevent such incidents. Safety comes first. If the engine stalls, don’t panic—applying strong pressure on the brake pedal will always help you regain control.
You asked why the brakes don't work after the engine stalls. I've studied automotive principles: while the engine is running, it uses the intake manifold to create vacuum stored in the brake booster reservoir. During braking, the booster amplifies your foot force, making the hydraulic system operate easily. Once the engine stops, the vacuum source is cut off, and the remaining vacuum in the reservoir dissipates within minutes, completely disabling the booster. The brake pedal then becomes as hard as an iron block, requiring you to stomp with full force to be effective. This design is common in mainstream vehicles and nothing unusual. Pure electric vehicles differ, relying on motor assistance, which might perform better. Engine stalling could be caused by ignition issues or insufficient battery power, posing risks. It's essential to develop a habit of checking the engine status regularly to prevent vacuum system leaks. In driving situations, if the engine stalls, don't rush to restart it. First, press the brake pedal hard to control speed, then safely stop and inspect.
