Rick's Free Auto Repair Advice

Uneven brake pad wear

What causes uneven brake pad wear

Diagnosing uneven Brake Pad Wear

If you’re doing a brake job or having it done professionally and learn that you have uneven brake pad wear, here’s what to look for.

First, let’s get out terminology straight:
The inboard pad is the pad that’s pushed against the rotor by the caliper piston. The outboard pad (on a floating caliper brake system) is the pad that’s pulled into the rotor by the movement of the caliper.

brake caliper, uneven brake pad wear

Brake caliper nomenclature

Here’s how a floating caliper works

uneven brake pad wear, brake job cost, cost to fix brakes, brake caliper

You apply the brakes. Brake fluid pushes against the piston (#1). The force pushes the piston out and rolls the square cut O-ring (#2) towards the rotor. The inboard brake pad hits the rotor (#3).

uneven brake pad wear, brake job cost, brake job caliper, warped brake rotor

Since the inboard pad can’t move any further, the pressure in the caliper bore forces the entire caliper to move in the opposite direction (#4). That pulls the outer edge of the caliper (#5) and the outboard brake pad (#6) against the rotor.

brake job, brake job cost, uneven brake pad wear, warped brake rotor

When you release the brakes, the brake fluid pressure releases and the square cut O-ring rolls back into place, forcing the piston (#7) back into the bore slightly. The brake pads move away from the rotor slightly

The charts below show different types of uneven pad wear and their associated causes.

After you correct the root problem replace and lubricate pad slide hardware. Examine the caliper slide pins and replace if you detect any corrosion. Lubricate the pad slides and caliper pins with a high temperature synthetic brake grease. Clean the rotor. Then install new brake pads and “bed” them by slowly stopping the vehicle from 30-mph. Allows 30-seconds for the pads to cool. Then repeat the procedure 29 more times.

Condition 1 Excessive inboard pad wear

uneven brake pad wear, brake pads wear too fast

Inboard pad is worn more than outboard pad

Here’s a situation where the inboard pad is worn more than the outboard pad, but the worn friction material isn’t tapered. The friction material thickness is approximately the same on the left/right and top/bottom of the worn pad. The following items can cause this kind of inboard pad wear:

Possibility #1
The square-cut O-ring seal has hardened, preventing the piston from retracting to the “at rest” position. That causes the caliper to continue to press the inboard pad against the rotor.

SOLUTION:
Rebuild the caliper with a new O-ring, dust seal, and slide bushings or replace with a factory rebuilt caliper.

Possibility #2  Corroded caliper guide pins
The square cut O-ring has retracted but the caliper guide pins are corroded or partially seized. In this case the caliper applies pressure, but the corroed slip pins prevent full release of pressure on the inboard brake pad.

 

 

 

 

Replace corroded caliper guide/slide pins with new parts. Lubricate with high temp synthetic brake grease

SOLUTION:
Clean and lubricate caliper guide pins and bushings with a high-temperature synthetic brake grease. If the corrosion can’t be removed, replace corroded pins. If the pin bores are corroded, replace the caliper bracket

Possibility #3
The brake pads are seized in the anti-rattle clips or pad slides. The brake pad steel backing plate can no longer move away from the rotor due to the corrosion on the anti-rattle clips.

brake pad, brake job cost,

High heat degrades the spring tension of the anti-rattle clips and rust can cause the brake pad “ears” to bind

SOLUTION:
Replace pad slide hardware on both calipers and lubricate with proper lube. Replace the pads.

Possibility #4
The caliper piston has physical damage or is corroded.

SOLUTION:
Replace the caliper and pads.

Possibility #5
Residual pressure in the hydraulic system is preventing the caliper piston from retracting to its “at rest” position.

SOLUTION:
Check the condition of ABS hydraulic unit, master cylinder, metering, and proportioning valves. Replace parts as necessary.

Condition 2 Excessive outboard pad wear

brake job, uneven pad wear, brake pads are tapered

The outboard pad is worn more than inboard pad

The following items can cause outboard pad wear:

Possibility #1
The caliper body isn’t releasing clamping pressure on the outboard pad due to corroded or seized caliper guide pins.

SOLUTION:
Clean and lubricate caliper guide pins with high-temperature synthetic brake grease. Replace corroded pins.

Possibility #2
The outboard brake pad has seized in the slide.

SOLUTION:
Replace pad slide hardware on both calipers and lubricate with proper lube.

Condition 3 Pads are worn in a tapered or wedge pattern

The brake pads are worn in a tapered fashion

The brake pads are worn in a tapered fashion

The following items can cause tapered pad wear:
Possibility #1
The brake pads had to be forced into the caliper slide area. This is almost always caused by rust buildup on the caliper bracket or abutment that prevent proper installation of the brake pads. The binding prevents one end of the brake pad from sliding and keeps the opposite side in contact with the rotor.

SOLUTION:
Double-check the pad fitment. Remove the pads and check for corrosion on the ends of the pad backing plate. Check for debris wedged in the pad slide area. Correct the condition and replace the pads. Clean corrosion off the abutment area with a wire brush and apply a light film of high-temperature brake grease to slow down future corrosion

Possibility #2
One caliper guide pin is worn or seized or the rubber guide pin bushing has deteriorated. Either condition would cause the caliper to cock during application and apply uneven pressure to the pads.

SOLUTION:
Check the guide pins and rubber bushings for wear, corrosion, incorrect lube, or lack of lubrication. Replace cracked or deteriorated rubber bushings. Install new guide pins and lubricate with high-temperature synthetic brake grease.

Condition 4 Pads are cracked, glazed, missing friction material, or have edge lift from the backing plate.

cracked brake pad

Brake pad friction material has cracked

The most common cause of cracked or missing friction material is poor quality backing plate steel that has rusted, thereby breaking the bond between the friction material and the backing plate.

When steel corrodes, the rust blooms expand and exert outward pressure on the friction material. The friction material is glued to the backing plate and cannot withstand the pressure. So the friction material is “rust jacked” away from the backing plate. Once that happens, brake pressure caused the friction material to crack and fall off.

For more information on steel backing plates, see this post

The second most common reason for broken or missing friction material is a cheap steel backing plate that flexes during braking. Cheap brake pads are made inexpensive steel backing plates that are too thin for the application. Since pressure is only applied to the center of the backing plate, the outer edges flex, causing the friction material to crack off.

The third most common cause of this condition is thermal distress caused by improper installation, driver overuse of brakes, residual pressure in the hydraulic system, or improper adjustment or operation of the parking brake.

Possibility #1 Backing plate corrosion

Corrosion on the backing plate has destroyed the glue bond between the plate and the friction material and the “rust jacking” has lifted the friction material up from the plate, where it breaks off.

SOLUTION:
Replace the brake pads with a higher quality set, preferably with galvanized steel backing plates.

Possibility #2 Backing plate flex

The backing plate is too thin for the application and has flexed while braking causing the edges to lift off the backing plate and break off.

SOLUTION:

Replace the brake pads with a higher quality set, preferably with galvanized steel backing plates.

Possibility #3 Thermal stress caused by overzealous braking or improper bedding
An improper bedding procedure involving an overzealous brake pedal application could cause the friction material to overheat, glaze, or crack.

SOLUTION:

Follow the brake pad manufacturer’s instruction for proper bedding. Avoid hard braking.

Possibility #5 Residual pressure in the hydraulic system is preventing the caliper piston from retracting to its “at rest” position.

SOLUTION:
Check the condition of ABS hydraulic unit, master cylinder, metering and proportioning valve. Replace parts as necessary.

Possibility #6
Improper fitment of pads, rotor, or shims.

SOLUTION: Double-check fitment and replace if necessary.

Possibility #7
If the pads are installed on the rear brakes that include an integral parking brake, the parking brake is not adjusted properly, the cable is seized, or owner drove while the parking brake was applied (rear disc brakes).

SOLUTION:
Check parking brake cable operation and adjustment. Adjust or replace parking brake components as needed. Replace brake pads

Possibility #8
Driver overused brakes. Check for signs of rotor or caliper overheating. Check with the customer to see if they haul heavy loads or a trailer.

SOLUTION:
Replace rotors and calipers if they show signs of overheating. Replace pads. Advise customer on proper vehicle and trailer load limits and proper braking technique when hauling heavy loads.

Possibility #9
Brake pads were defective or low quality

SOLUTION:
Replace with application-specific brake pads.

©, 2015 Rick Muscoplat

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