Torque Shop: Hissing noise when using the brakes is not normal and should be rectified immediately
Sign up now: Get ST's newsletters delivered to your inbox
Modern cars have a servo-assisted braking system to make the pedal less heavy to actuate.
PHOTO: ST FILE
Follow topic:
I recently purchased a six-year-old car, which makes a hissing sound when I apply the brakes. The salesman who sold me the car said it was normal. What is the cause of the noise and is it really normal?
The hissing sound when you press the brake pedal is a sign of a leak in the car’s vacuum line system. The typical braking system used in cars today has a “servo assistance” feature which boosts braking pressure when the driver steps on the pedal.
Commonly called the brake booster, the device uses engine vacuum or, in the case of electric vehicles, a pump to help the brake pedal motion. The device is usually connected directly to the brake pedal lever mechanism, and its sole function is to reduce the pedal effort required of the driver for safe and effortless braking.
A hissing sound whenever the brake pedal is depressed suggests a vacuum leak. If the sound is clearly from somewhere within the engine compartment, then there may be a broken vacuum hose or a defective valve that is connected to the servo unit.
If the hissing is very audible inside the car, then it is highly likely due to a punctured diaphragm.
A leaking vacuum tube or vacuum control valve at the servo unit is not expensive or complicated to repair. On the other hand, a punctured diaphragm means the entire servo unit has to be replaced as motor workshops do not service, repair or rebuild it.
Depending on the make and model of car, a replacement unit could cost several hundred dollars. If the leak is not fixed, it will worsen with time, eventually leading to a total loss of vacuum servo assistance and a heavy brake pedal feel.
This symptom does not imply that a total brake failure is imminent. Although the pedal becomes heavy, the brake system still functions.
It is, nonetheless, a major cause for concern as you will obviously not be able to apply the full braking force that the system is capable of. Braking response is also significantly impaired.
An even worse situation is if the heavy pedal happens only intermittently, so there are no warning signs that the braking is going to be heavy work. This can lead to an accident because the driver is not able to get the needed braking performance.
One thing is for sure. The problem will not go away on its own. So have the vacuum servo-assisted brake booster unit and system checked and rectify the situation immediately.
