When you step down on brake pedal, if is it fairly low then the brake pad wear sensor is the one making a squealing noise.
As indicated above, the brake pedal is low - this generally means the pads are low. Now, most brake pads have something called a wear sensor - a thin piece of metal connected to the pad. When the brake pads wear low enough, the wear sensor makes contact with the disc and consequently produces a squealing noise. When this noise is heard, it is an indication that the pads are getting extremely low and are in need of replacement.
What to do?
The car needs new brake pads. Take it to a shop and explain to the mechanic exactly what this article explained to you. New pads will both give you greater braking power and correct the squeal.
In addition to new brake pads, we recommend that you have the discs machined to ensure proper alignment with the new pads.
However, if when you step down on brake pedal, it is not fairly low then High-frequency vibration of brake pads causing a squeal.
If the wear sensor (i.e. low pedal and hence worn pads) is not causing the squealing sound, the noise is a result of the brake pads vibrating at a very high frequency (sort of like a tuning fork).
There are many things that can cause pads to vibrate but unfortunately, none of them are very easy to detect. You may want to remove the tire to expose the brake assembly where the squealing is coming from. Look for either loose brake pads or a loose caliper. Also, check to make sure the disc does not have deep grooves in it (i.e. "scoring") since these will usually cause enough vibration when the pads come in contact with the disc to produce squealing.
Additionally, check brake hardware such as anti-squeal shims and anti-rattle springs for looseness. Finally, a couple items which are hard to recognize but should be mentioned are glazed pads and non-factory pads.
What to do?
First, we should mention that brake squeal is often not an indication that one's brakes are shot. As long as the brakes are working well and the brake pads are not too worn, the squealing noise is just an annoyance. However, it is usually irritating enough to want to correct it. As a quick fix, you may want to visit your local auto parts store and ask the clerk what he/she has available to fix brake squeal.
How to Repair Brake Squeal noise
There are a few products on the market which are cheap, very simple to install/apply, and fairly effective in quieting squeal (by absorbing brake pad vibration). Alternatively, you may want to take this vehicle in for a brake inspection. Have the mechanic examine the items presented in the "Explain" part of this diagnosis.
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