How to understand, isolate, and repair disc brake problems.

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Click to enlarge image 1_diagnose-disc-brake-problems_master-cylinder-fluid-level_01.jpg

1) When diagnosing brake systems, check the fluid level in the master cylinder first. If the fluid level is down, it might be an indication of a leak somewhere in the system (although as pads and shoes wear, the fluid level does drop).

Squeak. Squawk. Squeal. Grind. It's the sound of brakes going bad. But how in the world do you know just what's going on with brakes by the sound they make? In some cases, the sounds are perfectly normal. In others, they can spell big trouble. This article will help you isolate and diagnose disc brake problems.

Automakers along with parts manufacturers typically incorporate audible brake warning devices into their brake systems. What this means is the brake pads are designed to squeal when the pads are at the end of their service life. When driving normally (foot off the brake), if the brakes emit a more or less constant squeal, it's time to replace the pads. In other more exotic applications, the pads have a warning light arrangement. A light on the instrument cluster will illuminate when brake pads need replacement. We should also point out that many disc brake calipers are engineered so that the amount of brake lining remaining on the pads can be visually checked and measured without caliper disassembly.

It's no secret that virtually all modern automobiles and light trucks on the road today are equipped with disc brakes at least on the front axles (front) or on all axles. The reality is, disc brakes function better than the older drum brakes, but they cost more to manufacture than the older drum arrangement. Because of this, you'll sometimes discover drum brakes on the rear on certain cars and trucks. In operation, disc brakes use the caliper to perform the same task as the wheel cylinder(s) in drum brakes; but, simultaneously, the caliper physically retains the brake lining (in discs, the lining is in the form of pads rather than curved shoes found on drums).

The rotor is a steel (iron or composite: iron/steel and/or aluminum) disc attached to or integral with the wheel hub. The rotor is machined perfectly flat on both sides. The caliper rides bridge the rotor so when the brakes are applied, the pads grip the rotor or "disc" between the respective "jaws," creating a huge amount of pressure (think of a big "C" clamp over a spinning rotor). The basic steps for diagnosing disc brake problems are the same as those for drum brakes.

The place to begin with disc brake diagnosis is to carefully inspect the master cylinder, lines and the calipers for leaks. If there are no visible leaks, proceed with the following:

Excessive Pedal Travel
Too much pedal travel can often be attributed to an air leak or fluid leak somewhere in the system or to fluid level that is low within the master cylinder. If the brake pads are not correctly installed, or if they have somehow become unseated, they can cause the same problem. If a set of rear drum brakes is out of adjustment, they can cause the same problem. Finally, if the wheel bearing adjustment is too loose, it can produce a similar symptom.

Disc Brake or Pedal Chatter
A bad rotor usually causes chatter or a pulsating vibration that is felt through the pedal when it's depressed. The rotor can become warped, damaged or worn so that it no longer has flat surfaces (on both sides) for the brake pads to grip. Another problem could be the front wheel bearings. If worn excessively, they'll allow the rotor to wobble (the rotor is either attached to the hub or integral with it, and it is in turn supported by the wheel bearing on the spindle). Should the hub wobble the sides of the rotor will not be flat to the surface of the brake pads. The result is a pulsating pedal or a brake pedal "chatter."

A dial indicator can be used to check for rotor "run out." The dial indicator has a sensitive gauge movement allowing the measuring tip to read differences of thousandths of an inch. To use it, the dial indicator is set up on a stand so that the tip of the gauge is placed against the surface of the rotor. As the rotor is turned by hand, the gauge will show any variations in the surface. This is known as measuring lateral run out. It is also possible to take a series of measurements around the rotor to determine the thickness (checking for high and low spots), but it is much more time consuming than using the dial indicator method.

Disc Brakes Pull to One Side or Uneven Braking
Pulling to one side, or uneven braking, can be caused by the pistons behind the pads becoming frozen by corrosion (or occasionally by dirt). This piston seizure prevents the pads on one caliper from touching the surface of the rotor and providing braking action. Please note that if one brake is not working (on given side), the car will pull toward the opposite side. Other likely causes of uneven braking are grease or brake fluid on the disc linings or rotor or a caliper that is loose on its mounts.

Excessive Brake Pedal Effort
If the brake pedal feels hard, this could be a sign that the pistons on both calipers are sticking due to corrosion, however the chances of this occurring on both sides of the car are slim. Instead, it is more likely that brake fluid or grease contaminating the pads created the problem. In addition, brake pads worm beyond their limit can also be a major cause. On some cars, the proportioning valve which is engineered to adjust braking pressure between front disc brakes and rear drum brakes could be malfunctioning. Also, because most disc brake systems require some form of power assist to make the disc brakes function effectively, high pedal pressures coupled with a reduction in braking force could indicate there is a problem with the power booster.

Disc Brake Groan
Squeaking or groaning disc brakes can be irritating, but the noise is not an indication of a brake problem. Do not confuse this with the squeal of pads that need replacing. Brake groan or an occasional squeal as the brake pedal is applied is different! Groan or squealing is sometimes caused (under certain pedal pressure conditions) by the rotor slipping between the brake pads. It can also be caused by brake pad dust build-up. There are some commercial fixes (in the form of aerosols) that can fix the annoyance, at least temporarily.

Disc Brake Rattle
Many brake pads incorporate a clip behind the pad. This is appropriately called an "anti-rattle" clip. The clip holds the pad under a slight spring tension in order to prevent noise when the pads are not in contact with the rotor. Incorrect pads for the application can also be the culprits, but that's highly unlikely.

No Braking Effect When Pedal Is Depressed
This is a serious issue. Do not drive the car until the problem is diagnosed and repaired! Complete loss of braking power is most often a sign of a ruptured line or faulty master cylinder or caliper piston. Check the level of the fluid in the master cylinder and carefully examine all of the lines and all calipers for evidence of leaking fluid. Additionally carefully inspect the bleeder screws on all calipers and wheel cylinders to ensure one (or more) has not somehow opened. Loss of a power-booster will not result in complete loss of braking.

Brakes Heat Up And Fail To Release
This is also a very serious issue, which may be caused by caliper pistons that have seized due to overheating (usually after heavy braking when descending a hill and/or towing a trailer). One other cause of this problem is a driver who rides the brake pedal with his foot while driving. In some cases, the pistons in the caliper will return to normal following a [long] cool down. In other more severe cases, the vehicle may require a major brake overhaul.

For a closer look, check out the photo gallery and captions to help you diagnose disc brake problems on your own car or truck:

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