If you own a high mileage vehicle, you may start to notice hissing noise coming from the brake pedal. The noise may be there constantly, or you may only hear it when you press the gas pedal. If you hear a hissing noise from the brake pedal area, it means that the brake booster is failing.
Air Noise from brake pedal
Hissing sound when applying the brakes
Brakes sound like air is coming out.
Brake booster problem hissing sound
Whoosh sound when you apply the brakes
It may sound like air is coming out when you press the brakes, but in the reality, air is being sucked in. The noise is caused by a leaking brake booster. A brake booster has a diaphragm inside on one side of it the vacuum created by the engine. The brake booster has a hose that is connected to the intake manifold of the engine. The vacuum created from the engine makes pressing the brake pedal easier and assists you so that you don't have to press the brakes as hard to stop the car.
Typically when you hear the hissing noise from the brake pedal you should check the brake fluid as well just to be sure you are not losing any. In most cases, you don't loose any brake fluid. The noise is caused by air being sucked via a cracked brake booster O-ring or silencing foam or diaphragm.
In some cases, you may notice that the noise goes away when you press the brake pedal or when you press the gass.
In most cases replacing the brake booster will fix this problem. Replacing the brake booster requires some DIY skills so if you don't plan on fixing the hissing noise yourself take you car to a mechanic.
The brake booster is located right behind the master cylinder. It is the black round tank between the master cylinder and the firewall.
Don't neglect the hissing sound coming from behind the dash/brake pedal area. If you continue to neglect it, you may loose your brakes while driving which can be catastrophic.
Bad Brake Booster Diaphragm
The diaphragm that moves as you press the brake pedal may have a tear allowing air to be sucked in. If the diaphragm is damaged it will be harder to press the brake pedal.
Failing Brake Booster
The brake booster has several O-rings and also a foam to make it quieter. The O-rings are to isolate the chambers of the brake booster. The foam purpose is to quieten the brake system operation. If any of these get damaged you will hear that hissing noise when you press the brakes.