Why Does My Car Shut off When Braking?

The most likely cause of this is a leak in your vacuum booster.

What Is a Vacuum Booster?

To understand what a vacuum booster is, let’s quickly go over how exactly a power braking system works. Power brakes need two main components to function; the vacuum booster (or brake booster) and the master cylinder.

The vacuum booster uses either a pump or a tube connected to the intake manifold to generate a vacuum inside the booster’s chamber. This booster is also connected to a diaphragm that leads to the master cylinder.

When the brake pedal is pressed, it opens a valve that allows air into the vacuum booster. This increased atmospheric pressure moves the diaphragm towards the master cylinder with more force than was used to operate the brake pedal.

The master cylinder is, in turn, filled with brake fluid and connected to the brake lines that lead to the calipers. When the diaphragm moves into the master cylinder, it generates hydraulic pressure within it. That pressure is sent down the brake lines where it forces the calipers onto the brake discs, slowing the car down.

The main takeaway here is that in the case where your vacuum booster gets vacuum from the intake manifold, any leak in your booster will cause your engine to shut off.

Why Would a Bad Vacuum Booster Shut Off My Engine?

A leaky vacuum booster, while not quite as serious overall, has some of the same symptoms as a leaky head gasket, namely power loss, engine misfiring, and a tendency to stall. Because vacuum lines connect the booster to the manifold, a leak in the booster has the same immediate results as a leak in the manifold.

The intake manifold needs to be sealed tight to ensure that no extra air gets in during the combustion process. If this occurs, the engine will begin to run lean, resulting in reduced power.On occasion, a leaky brake booster can result in brake fluid being drawn into your vacuum lines, which you don’t want as it can cause the lines to corrode.

If too much air gets into the intake manifold and there’s not enough fuel to balance it out, then there’s a pretty high chance that your engine will stall, especially at a lower RPM. That’s why a leaky vacuum booster tends to make engines stall during braking.

If you think there’s a leak in your vacuum booster and want to check for yourself to see if that’s what’s stalling your engine, one way to test this is to use something to tie off the vacuum lines connecting the booster to the manifold. If the booster is indeed the problem, this will prevent excess air from leaking into the manifold.

The only caveat to testing your vacuum booster this way is that by blocking off your vacuum booster from the intake manifold, you’re essentially disabling the “power” part of your power brakes. Your brakes will still work, but you’ll have to apply a lot more pressure to the brake pedal for anything to happen. 

If you’re planning on using this method to check for leaks in your vacuum booster, make sure you do it in a safe place with a lot of room, like a deserted parking lot.

Are There Other Possible Explanations?

While a leak in your vacuum booster is most likely the reason why your car is shut off while braking, there are a few other possible causes.

One possible cause of this issue could be a bad idle air control (IAC) valve. This valve bypasses your car’s throttle and allows air to enter the combustion chamber at idle when the throttle is closed. If your IAC valve gets gunked up with carbon deposits, it could get stuck shut, resulting in your engine stalling when it returns to idle.

It’s possible that a faulty fuel pump could be the cause of the issue as well. A broken pump won’t be able to deliver consistent fuel pressure, which can cause the engine to stall at low speeds.

In cars with automatic transmissions, a bad torque converter or a low level of transmission fluid can also cause your engine to stall at low speeds since the transfer of power from the engine through the transmission becomes interrupted. 


While there are a few possible causes of why your engine is shutting off when you apply the brakes, the most likely reason is probably a leaky vacuum booster. Luckily, it’s pretty easy to be able to check if this issue is indeed being caused by your booster. 

A leaky vacuum booster probably won’t cause any serious damage to your engine. However, it’s not safe if your car is constantly stalling at low speeds, so if this is happening to you, then it’s essential to get your car fixed as soon as possible.