Disc brakes in a car: How they work, common problems

Updated: May 24, 2020
Disc brakes
Disc brake components

All modern cars have disc brakes on front wheels and disc or drum brakes on the rear axle. How do disc brakes work?

The main component of the disc brake setup is a brake caliper. The hydraulic piston(s) inside the caliper can apply clamping force to the brake disc using two brake pads. The clamping force can slow down or stop the rotation of the disc and the wheel; see the diagram.

Passenger cars, SUVs and pickup trucks on the road today have a hydraulic braking system filled with brake fluid. When the driver presses the brake pedal, it pushes the piston inside the brake master cylinder that creates pressure within the hydraulic braking system. The brake fluid transfers the pressure to the brake calipers that apply brakes to the wheels. Let's look at the disc brake parts.

Brake disc (rotor): The brake disc (rotor) is mounted on the hub and rotates together with the wheel.

Brake disc and pads

Brake disc and pads.

During braking, kinetic energy of the vehicle converts into heat due to friction between pads and the disc. To better dissipate the heat, most vehicles have ventilated discs on the front wheels. Rear discs are also made ventilated in heavier vehicles. Smaller cars have solid (non-ventilated) discs.

Brake pads: Brake pads are installed within the brake caliper, on both sides of the brake disc so that they can slide laterally, towards and away from the disc.

New set of OEM brake pads

New set of OEM rear brake pads.

A brake pad consists of a friction material pad molded to a metal backing plate, see the photo of Toyota front brake pads. In many brake pads, noise-reducing shims are attached to the backing plate.

Brake pads wear out with mileage. To work properly, the pads have to have enough friction material. Car manufacturers specify the minimum thickness of the friction material (lining). If any of the pads are worn out close to that limit, or there is any damage to the pads, all brake pads of the axle must be replaced. See the photo of a brake pad that is worn out beyond the limit.

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Brake caliper: There are two types of brake calipers: fixed calipers and floating calipers. Fixed calipers are often installed on luxury and sports cars. Most of the vehicles on the road today have floating brake calipers similar to the one in the photo. A typical floating brake caliper has one or two pistons on the inner side.

Brake disc and pads

Floating brake caliper.

Fixed brake caliper

Fixed brake caliper.

Compact cars and SUVs typically have single-piston calipers, while larger SUVs and trucks have dual-piston calipers on the front axle and single-piston calipers at the back.

Front brake calipers are mounted on the steering knuckles. Rear calipers are bolted to the rear spindles or the rear axle. In the most common design, the lubricated caliper guide pins allow lateral movement of the caliper. This movement is vital for proper brake operation. Seizing up of the caliper pins will lead to many problems.

How long do disc brakes normally last?

As a result of the friction, brake pads and rotors wear out. In an average car, disc brakes normally last for 30,000-50,000 miles. Worn out pads need to be replaced. Rotors can be resurfaced or replaced depending on their condition. Replacing brakes (pads and rotors) on one axle costs from $410 to $650 for an average car. Of course, there are many reasons why brakes can wear out prematurely.

Common problems with disc brakes

Brakes are vital for safe driving. Problems with brakes make the vehicle unsafe to drive. Common problems with disc brakes include:
Noises: Disc brakes can cause a variety of noises. Brake pads have metal wear indicators that are designed to make a metal squealing noise when brake pads wear out close to the limit. Rust buildup on the edges of the working surfaces of the rotors can cause squeaking noises. If the noise is confirmed to be caused by rust buildup, servicing brakes might be enough if the pads and rotors are still in good shape.
Poor-quality brake pads can also produce squeaking while braking. In some cases, squeaking can be reduced by applying a thin layer of special noise-reducing paste or grease to the back of the brake pads. Of course it must be done following manufacturer's instructions with an approved product. With any brake noise, we advise having your brakes checked as soon as possible to keep your car safe.

Warped or unevenly worn rotors: As a result of the excessive heat, rotors can warp. Mostly it happens to front rotors in heavier vehicles.

Disc brakes
Technician checks the rotor for uneven wear or warping.

The main symptom of warped front rotors is shaking of the steering wheel that is noticeable when braking at higher speeds.
Rust on the rotors can cause the same problems. When the car is parked for days without driving, sections of the rotors rust. This causes rotors to wear unevenly: sections that were damaged by rust wear more and become thinner. The result is the same: shaking of the steering wheel and vibration felt in the brake pedal when braking at higher speeds.

The solution is to have the brakes inspected. Mechanics have special tools that can measure if the brake rotor is warped or worn unevenly, see the photo. Typically warped or unevenly-worn rotors need to be replaced or machined, depending on their condition. Brake pads might also need to be replaced if they don't pass the inspection. If the pads are in good shape and pass the requirements, a mechanic may advise replacing rotors only and servicing brake components.

Rust damage of the brake disc

Rust damage of the brake disc.

Rust damage: This problem is very common in the salt belt. Brake rotors and other parts tend to rust when the car is parked outside for some time. Road salt accelerates the corrosion process and not much can be done to stop it. For example, check this photo: Rotors in this car got rusted while it was parked. Only about half of the working surface of the brake disc shows a sign of contact with the brake pad. This disc fails the manufacturer inspection requirements and must be re-surfaced or replaced.

Sticking or seized brake calipers or pads: Brake calipers (pistons or guide pins) tend to stick and seize up, especially if brakes are not serviced regularly. Brake pads can also seize up inside the caliper or inside the caliper bracket.

New brake caliper

New rear brake caliper.

If caught early, servicing brakes may be enough to restore operation of the brake calipers. In some cases, the caliper will have to be replaced.
Symptoms of a sticking brake caliper or pads include dragging brakes, squeaking while driving, overheating of one of the wheels or excessive brake dust on one of the wheels. In a sticking brake caliper, brake pads tend to wear unevenly: one of the brake pads might be worn more than other pads. In some cars, sticking rear brake calipers can cause parking brakes not to release fully.

Often, a bad or failing brake caliper doesn't show any symptoms and can only be caught during an inspection.

Bad brake caliper

Bad brake caliper.

For example, the broken piston boot in the brake caliper in this photo caused internal corrosion of the piston. Over time, a corroded piston will seize up or start leaking brake fluid. This caliper must be replaced as a precaution even though it still works. This is one of the reasons why regular brake inspections are important. During an inspection, your mechanic may catch the problem early, when servicing brakes without replacing any parts might be enough. If the brake caliper is found to be bad, it must be replaced. Replacing one brake caliper in an average car or SUV costs from $320 to $650. There have been several recalls related to bad brake calipers, check with the dealer or on the NHTSA website.

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How much does brake service cost?

Servicing disc brakes in an average car without replacing parts costs from $90 to $170 for one axle. Servicing brakes involves taking them apart, inspecting and cleaning the excess rust and lubricating areas that must be lubricated with special grease. If the rotors require re-surfacing, it will cost more. Brakes need to be inspected regularly and serviced if needed. Check your vehicle's owner's manual for details. In the 'salt belt', brakes need to be serviced at least once a year.

Regular brake inspections

Twisted brake hose
Twisted brake hose.

Properly functioning brakes in your car can be the difference between stopping in time and having an accident. Even if they seem to work properly, we strongly recommend having brakes inspected at least once a year or earlier if so advised in the maintenance schedule for your vehicle. Many problems with brakes can be caught early. For example, in the car in the photo, the brake caliper is not installed properly, causing the brake hose to twist. To avoid situations like this, trust your brakes only to a qualified professional.
Your brakes also need to be inspected if you suspect a problem with them, for example, when you notice noises coming from the wheels or when brakes don't work as they should or when one of the wheels appear hotter than others or when the car pulls to one side while braking.

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