Disc brake service is one of the most commonly performed DIY jobs.

© DenLorsTools.com Summary: Disc brake service is one of the most common jobs done by the weekend mechanic. Find out how to tell when brake pads need to be replaced, it’s simple after you know what to look for. Most people are surprised to find out that brake noise is one of the most common causes of comebacks that professional car repair shops deal with. Learn in this auto repair blog which types of cars are more prone to having squeaky brakes than others. Find out what some car owners do that causes brake squeak that they are not even aware of! Also learn what the professionals do to reduce brake noise to help prevent customers from having to bring their cars back for warranty repair.

How to tell when brake pads are worn out?

Most brake pads are considered worn out when the friction material is at or below 3/32″. The metal backing plate is NOT a wear part and isn’t measured when checking the brake pad thickness. Many brake pads have a metal warning sensor that is designed to touch the brake rotor once the friction part of the pad (also called the brake lining) is worn down to this minimal thickness. The result of the metal warning sensor touching the brake rotor while it is turning is a very loud and high pitched noise. Even if the driver knows nothing about brakes, they will surely hear the noise and have it investigated. Some car owners or “buy here pay here” car lots have been known to bend or remove the warning clip to squeeze a few more miles out of the brake pads; this is flirting with disaster since rapid damage can be done to the brake rotors if the worn out brake pads are allowed to wear into the rotors. If this results in metal to metal wear, a much more expensive brake job will be needed.

Some cars have an electronic warning that displays a message on the dash.

This type of warning system takes the guess work out of when the brake job is needed. It indicates right on the dash when the “brake pads are worn.” The message on the Mercedes above tells the driver that brake pads are worn and to visit a workshop. (Workshop is the European equivalent to “auto repair shop” in the USA.) Once the sensor is worn into, it must be replaced when the brake job is performed or the warning message will stay on. This type of warning should not be confused with a brake light on the dash that indicates the parking brake is on or that the fluid level is low in the master cylinder. Also if the brake light comes on at the same time the brake pedal sinks to the floor, this indicates a hydraulic failure – and is most likely an internally by-passing master cylinder.

New brake wear sensor pictured above with new pads and rotor.

The sensor above completes a circuit which makes the warning stay off. If an old sensor is reused the warning message will stay on. Some brake pads will come with new sensors and others will not. This seems to vary depending on the manufacturer. Be sure to check to see if a brake sensors (or sensors) are included with the brand of brake pads you purchase. Most of the time they will need to be purchased separately.

Where does the squeak or “squeal” come from?

1. Assuming that the brake pads have more than 3/32 of thickness on ALL pads, read the following; Brake squeaks and “squeals” can come from the friction part of the pad or from the metal backing – meaning the front of the pad or the back. Brake squeaks or rattles that occur while driving and NOT applying the brakes, is usually from brake pads moving excessively. If the noise stops when the brakes are applied this helps to determine that this is the case. When the brake pads are squeezed, excessive movement is most of the time eliminated. Brake pads can move excessively if there are missing anti-rattle clips or if the brake pad shims (or noise insulators) are missing or are allowed to move back and forth.

The fix is to replace any missing anti-rattle clips and secure any shims on the back of the pads. Disc brake quiet can be used on the back of the pads. Disc brake quiet is like a glue that helps secure the pads to the caliper and caliper piston. Alternatively, brake lube or silicone paste like Sil-Glyde can be used on the backside of the pads and on the contact points. This method does not adhere or insulate – instead it lubricates much like oiling a squeaky door hinge. *If using disc brake quiet, brake lube or Sil-Glyde be sure to follow directions carefully and DO NOT allow any of these products to get on the friction part of the pads.

2. If the brake squeal occurs when the brakes are applied, then it’s usually a friction surface issue. Meaning the noise is coming from the act of the front part of the pads squeezing on the rotors. Usually it’s because of improperly glazed pads and/or rotors. Also contaminated pad linings can cause brake squeals. How do they get contaminated? Many ways, read on.

a. If the brake rotors are not cleaned thoroughly when a brake job is performed (after machining), small metal particles can be embedded into the new brake pads causing noise as the metal rusts and deteriorates. The best way to clean them is NOT brake cleaner – it is with plain soapy water. Wash them off with dish soap water and a brush, then thoroughly rinse.

b. Disc brake quiet or brake lube allowed to get on the friction surface of the pads. Wheel bearing grease used to be a common source of contamination back when most disc brake rotors were held on with serviceable wheel bearings. Most cars today have slide on rotors and the wheel bearings are not serviceable, so this is not as common nowadays.

c. Car owners may be causing the squeal problem unknowingly. Many wheels today have large openings that can allow over-spray from “Tire Shine” to land on brake rotors. When the car is driven the tire shine ends up contaminating the brake pads.

The fix is to resurface the pads, or replace them and swirl or machine the rotors. Caution should be taken not breathe the brake dust. There may or may not have asbestos in the pads but one thing for sure is you don’t want dust of any type in your lungs. One method to scuff or resurface the brake pads is to use the concrete driveway as the sand paper. Taking off the glazed or contaminated portion will many times reduce the noise. Ninety degree die grinders are most commonly used by techs to swirl the rotors. Many times this can be done right on the car. Spinning the rotor by hand or by putting the car in drive provides the motion that results in a nice even swirled surface. Obviously care must be taken around moving parts or personal injury can occur. Also it should be noted that pressing the brake pedal down with the caliper off may over-extend the brake caliper pistons. This may cause a leak and a possible caliper rebuild or replacement to be necessary.

Which cars are more prone to have brake squeaks?

It seems that the cars most likely to have brake squeal issues are Nissan, Infiniti, Toyota and Lexus. Some European cars also have their issues. In these cases it may be best to stick with original pads and shims since aftermarket brake pads seem to cause more noise on these cars. As an alternative, ceramic pads may work well depending on the application.

Question: Should I bevel brand new brake pads before installing them?

Answer: I have had good success by beveling brake pads, although this method can be debated. The engineers have designed brake pads to be a certain size and configuration and by beveling the pads this does alter that. A Honda tech I worked with before used to always remove the paint from the friction part of the pad. He didn’t bevel it, he just used a die-grinder with a scotch disc to sand away the paint that was on the friction part of the pads. The reasoning is that the paint can heat up and curl. When it is curled somewhat, it can drag across the rotor like fingernails on a chalk board.

Question: Why do my the brake pads wear unevenly?

Answer: The most common cause of premature brake wear is due to the caliper slides sticking or being frozen up. It’s always a good practice to make sure the slides move freely. They should be taken apart and lubricated with a product like Sil-Glyde paste. Even if they move freely when doing the brake job, it’s a good idea to lube them because this will help to insure that the slides will continue to work freely for the life of the brake pads. Also make sure that excessive paint on aftermarket pads are not going to prevent them from sliding in the caliper bracket freely. It may be necessary to clean them up a bit with sand paper or a scotch disc on the metal contact points where the touch the caliper bracket.

Question: Should I go easy on the brakes right after a fresh brake job?

Answer: Yes. For the first couple of hundred miles you should try to brake lightly and allow the pads to… well “break-in.” Sorry for the pun. Some service manuals actually allow the technician time to burnish the pads. With competitive prices on brake jobs this is hardly ever carried out. Usually technicians will test drive the car only far enough to make sure it is stopping safely. Burnishing the brake pads usually occurs when the owner is driving if they know it or not. Braking too harshly may cause the pads to overheat which can cause excessive glazing resulting in unwanted brake noise.

Question: I just had my brakes done and they squeak every morning for the first few times I apply the brakes. The repair shop says this is normal. Are they trying to get out of fixing my brakes properly?

Answer: With most cars it is normal to hear some brake noise, especially in the morning the first few times the brakes are used. Remember the part in the article about high humidity climates causing surface rust on the rotors. If the car is parked outside more humidity from morning dew will like have more of an effect.

Question: Should I machine my brake rotors when doing a brake job or just put brake pads on?

Answer: Oh yes the famous “pad slap.” A pad slap is what most techs call an inferior brake job which consists of nothing but slapping pads on. Many times when the customer does this, they will end up bringing their car to the shop because of the noise it now makes. With that being said, there are different schools of thought on this. Service manuals will say not to machine rotors in many cases. My experience is that you will almost always have too much noise if the rotors are not machined when doing a brake job.

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