Because brake pads and rotors can be expensive, it's natural to try to get your full money's worth out of them. Letting pads wear down to the point where the metal backing plate grinds against your rotor will definitely require rotors to be replaced, and it may cause damage to brake calipers if their pistons have slid out too far during the process of squeezing brake pad material that's no longer there. We've grouped together some telltale signs to help you recognize symptoms of various problems, and know when various brake components are worn out and in need of attention.
It's time to replace brake pads when…
Squealing noise upon brake application is actually caused by a high-frequency vibration of metal rotors, drums, or brake pad backing plates. Excess corrosion that forms over time on non-contact, outer perimeter areas of rotors and drums is a prime cause because rust is looser and less dense in nature - therefore, more likely to create resonation.
Most brake pads are equipped with sensors built in to the brake pad material. When a brake pad that's 12 mm thick new wears to approximately 3 mm of pad life remaining, the metal sensor usually becomes exposed and makes contact with the disc. The resultant metal-on-metal contact causes squealing and may trigger an electronic sensor to display a warning light on newer vehicles. Either way, the brake pads themselves are worn to about 25% and will need replacement soon, depending on how brake-intensive your driving is. Gauge how many miles it took you to reach this point, and divide it by three to get a rough idea how much more time until ‘pad zero' is reached. At 3 mm thickness remaining, you've got a little time left. At 1 or 2 mm, it's time to schedule an appointment or order the parts to have at the ready for a specific future date if you're doing them yourself.
While eyeballing the thickness of your brake pads is a good way to keep on top of things, it's important to remember that there are outboard pads on the side of the rotor you can see easily, and inboard pads on the other side of the rotor that aren't visible without wheel removal or climbing underneath. Depending on vehicle application, inboard pads may wear at a faster rate, so the side you see through the wheel spokes cannot be fully trusted as a complete measurement of remaining lifespan. It's also important to note that putting a set of new pads on deeply ‘dished in' rotors may cause the edge of the new pad to rub the lip, causing more squealing noise than with the previous sets of pads.
Clicking noise from the wheels after depressing or releasing the brakes indicates pads are shifting around improperly. There's no danger of them falling off the vehicle, but because pads are under pressure between the caliper and brake rotor, built in ‘anti-rattle' clips lock the pads in place. Over time the steel these spring clips are made from can become brittle and break, allowing the brake pad to ride loosely in its seat, creating vibration and clicking noises during braking. When brake pads have seen enough use for this to happen, it's likely they are also well-worn out (often unevenly as a result) enough to need attention soon, if not immediately.
It's time to replace brake rotors when…
Another prime source of squealing is a lip that forms along the outer edge of rotor surfaces because brake pads do not make contact with that area. The lip becomes more pronounced as rotors wear and take on the shape of deep-dish frying pans. Sometimes, the lip becomes pronounced enough to cause contact with brake pads that aren't designed with chamfered (rounded) corners - with squealing as the natural result. Extremely cold outside temperatures which cause rotors to contract slightly can aggravate this effect, and getting the brakes extremely hot under repeated heavy use can lead to metal expansion which also causes more contact where it shouldn't be.
If you notice a shimmy that occurs only upon slowing down, your brake rotors need attention. In most cases, the shimmy is a result of rotors becoming warped similar to the way an old vinyl record can. With each rotation of the wheel, the warped part of the rotor pushes against pads in a side-to-side motion. This shimmying vibration is transferred from brake calipers directly through wheel hubs, axles, suspension subframes, and the vehicle frame itself to where it's felt. A quick way to be sure if the vibration is coming from front rotors is if the steering wheel also shimmies in unison with the brakes. Vibration felt only in the seat of your pants typically indicates just the rear rotors are warped. Rotor shimmy can also be caused by spots of corrosion that become embedded into the rotor surface when a vehicle sits in a moist environment for a lengthy period. Additionally, heat spots or glazing can form on rotor surfaces after the iron compounds in the rotor change composition due to extreme and repeated heat buildup.
To get rid of warping, an edge lip, or spots, rotors can be taken off the vehicle and ground flat on both sides by a special lathe. Because this resurfacing process involves removing a good deal of thickness across the entire rotor surface, it's important to be aware of what your vehicle manufacturer's minimum rotor thickness specification is. This is the point where the rotor still contains enough metal to absorb and diffuse heat buildup without becoming warped, glazed, or fractured. If rotors will be below minimum thickness after re-surfacing, they need to be replaced - mainly because they will quickly become warped again.
Other brake components need addressing when…
If your vehicle pulls to one side during heavier braking, the brakes are grabbing somewhere on one side of the vehicle more than the other. Most often, this is caused by calipers (or wheel cylinders on drum brakes) that have become corroded and no longer move properly to apply pressure on brake pads. Pulling can also be caused by a brake fluid leak onto a rotor or drum surface. Since brake fluid is not designed as a lubricant, it creates a sticky surface that creates more friction and causes a brake on one side of the vehicle to pull harder than the other. Brake proportioning valves that direct brake fluid pressure in the system might also be to blame, but the likelihood of that is lower.
If the brake pedal goes to the floor, fluid compression is not building up properly because of a leak somewhere in the system. If a visible brake fluid leak cannot be located at any of the wheels or along the brake lines, the master cylinder assembly located under the hood is most likely the issue. It does most of the compressing of brake fluid. Most often, internal seals go bad and reduced compression occurs inside the master cylinder with no visible leaks. If the brake pedal becomes extremely hard to depress, your brake booster is compromised. With either scenario, immediate attention is required and the vehicle should be towed to where it will be worked on.