DIY

How to Diagnose Faulty Front Brake Discs

Reading Time: 3 minutes

Rotors don’t take fancy equipment to diagnose. Most professionals test-drive the vehicle to see how the brakes feel. Then they perform a visual inspection of the rotors and, if necessary, measure the rotors as well. That’s it.

Front Brake Disc Inspection Methods

Rotors are simple, disc-shaped slabs of metal. As such, they’re usually pretty easy to diagnose.

Note: The following are general guidelines for educational and entertainment purposes only. Consult your vehicle’s factory information for specific repair instructions and recommended safety procedures.

1. Go by Pedal Feel

Pulsation or vibration while braking is almost always caused by warped rotors. There are other possibilities, such as a bad wheel bearing, but rotors are to blame most of the time. So, if your brake pedal pulsates while braking, you might want to just wing it and throw on a new set of front pads and rotors.

2. Visual Inspection

Take a look at your rotors: If they have deep scoring, extreme heat discoloration, hard spots, or cracks, it’s time for a new pair.

You’ll need a few special tools, such as a brake rotor micrometer and a dial indicator, to measure the thickness of your brake discs.

3. Take Measurements

If you want to be sure your rotors are worn, there are several measurements you can take. You’ll need a few special tools, such as a brake rotor micrometer and a dial indicator, to get the job done. Many professionals only take the time to measure thickness, but if you’re feeling ambitious, you can check all three measurements.

Obviously, you’ll need to raise and support the vehicle safely, then remove the wheel/tire assembly to perform these inspections. And don’t forget to put on your safety glasses. After you get the vehicle prepared, here’s what you can check:

Checking for minimum thickness is easy. Simply use a brake micrometer in the middle of the rotor friction surface (where the pad makes contact). Note the reading: If it’s below the manufacturer’s specification, the rotor should be discarded.

  1. Remove the caliper mounting bolts. Do NOT unbolt the brake hose going to the caliper. You’ll let air into the hydraulic system if you do.
  2. Free the caliper by pulling it up and away from the rotor. If the caliper is stubborn, you may need to use a large screwdriver or pry bar to help free it.
  3. Use either a zip tie or a bungee cord to secure the caliper to the steering knuckle or another sturdy suspension component. Do not allow the caliper to dangle from the brake hose—damage may result.
  4. Remove the brake pads from the caliper mounting bracket (floating calipers only).
  5. Secure the rotor to the hub with the lug nuts.
  6. Attach the clamp end of the dial indicator to a sturdy, fixed location, such as the steering knuckle or caliper mounting bracket.
  7. Place the pointed end of the tool at the center of the rotor’s surface (where the pad would ride) and set the dial to zero.
  8. Turn the rotor by hand while keeping the tool perpendicular to the rotor surface.

Note the reading on the dial indicator; as a rule of thumb, runout should be less than .002 inch. It’s a good idea, however, to compare your reading to the manufacturer’s specification to be certain. If runout exceeds specification, the rotor should be resurfaced (if possible) or replaced (recommended).

If you decide to give it a try, measure the rotor thickness in at least eight different places [2] around the rotor. Then subtract the thinnest measurement from the thickest—that is your reading. Compare your reading to the manufacturer’s specification to see if your rotor should be resurfaced (if possible) or replaced (recommended).

If you find the rotors are worn beyond specification, you’ll want to service them right away. On the other hand, if the rotors look good, you can put everything back together. Be sure to torque both the caliper mounting bolts (if removed) and the wheel lug nuts to the manufacturer’s specifications.

Caution: Once everything is reassembled and the car is back on the ground, pump the brakes several times to ensure they feel firm. DO NOT drive the car until the brakes feel solid.

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Author

Mia Bevacqua

Chief Mechanic at CarParts.com

Mia Bevacqua is an automotive expert with over 15 years of industry experience. She holds ASE Master, L1, L2, and L3 Advanced Level Specialist certification, as well as a bachelor's degree in Advanced Automotive Systems.

Throughout her career, Mia has applied her skills toward automotive failure analysis inspections, consulting, diagnostic software development, and of course, freelance writing. Today, she writes for companies around the world, with many well-known clients showcasing her work.

Mia has a passion for math, science, and technology that motivates her to stay on top of the latest industry trends, such as electric vehicles and autonomous systems. At the same time, she has a weakness for fixer-upper oddballs, such as her 1987 Chevy Cavalier Z-24 and 1998 Chevy Astro Van AWD.

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