Diagnosing brake problems

Failure modes of brake discs and brake pads

The following technical information and practical tips have been compiled by HELLA in order to provide professional support to vehicle workshops in their day-to-day work. The information provided on this website is intended for suitably qualified personnel only.

Failure modes of brake pads

Friction lining comes loose from the base plate

The friction lining has come loose from the lining's base plate, either in full or in part.

Cause

Thermal or mechanical overload
Subsurface corrosion between lining and base plate
Insufficient fitting accuracy of the brake pads
Damage to the brake pad due to incorrect handling or installation
Non-compliant brake pads

Effect

Grinding noises when not pressing the brake
Squeaking during braking
Deficient braking characteristics
Breakdown of friction lining due to thermal overload

Remedy

Clean the brake caliper and check for proper function ✓
Check the brake caliper and pad for fitting accuracy ✓
Replace the brake pads and accessory parts if necessary ✓

Thermal overload

Brake pad has been damaged due to overheating. Binder in the pad has been destroyed and the pad material is breaking down. The surface is porous and glazed in parts.

Cause

Sticking or tight guide sleeves
Brake piston stuck within the brake caliper
Extreme driving conditions or sustained braking
Sticking brake pad

Effect

Noises
Poor braking function

Remedy

Clean the brake caliper and check for proper function ✓
Check the brake caliper and pad for fitting accuracy ✓
Replace the brake pads and accessory parts if necessary ✓
Check the brake discs and replace them if necessary ✓

Grooves in the friction lining

The pad surface has deep grooves and wear edges.

Cause

New pads have been installed on the old, worn brake discs
Foreign bodies between the brake pad and disc
Environmental influences (salt, dirt, etc.)

Effect

Noises when operating and not operating the brake
Poor or one-sided braking function

Remedy

Clean the brake caliper and check for proper function ✓
Replace the brake pads, accessory parts and brake discs ✓

Material and edge chunking on the friction lining

The friction lining is damaged. The lining material has broken off at the edges.

Cause

Incorrect installation
Incorrect, non-compliant brake pads
Lack of freedom of movement for the brake caliper
Mechanical overload
Thermal overload

Effect

Noises
Poor braking function
Premature wear

Remedy

Clean the brake caliper and check for proper function ✓
Replace the brake pads ✓
Check the brake caliper and pad for fitting accuracy ✓
Check the accessory parts and brake discs and replace them if necessary ✓

Excessive wear

Brake pads are heavily worn and have worn down to the base plate in some cases.

Cause

Deficient maintenance
Exceeding the inspection intervals
Sustained braking during downhill driving
Sticking or tight guide sleeves
Brake piston stuck within the brake caliper
Extreme driving conditions or sustained braking
Sticking brake pad

Effect

Noticeable noises when operating and not operating the brake
Poor braking characteristics
Damage to other wheel brake assemblies due to thermal overload

Remedy

Disassemble the brake system and check it ✓
Clean the brake caliper and check for proper function ✓
Check the caliper guides, dust covers and damping sleeves and replace them if necessary ✓
Replace the brake pads, brake discs and accessory parts ✓

Uneven lining wear

The brake pads on one side of the wheel brake have been worn down unevenly.

Cause

Sticking or tight guide bolts or caliper guides on the brake caliper
Brake piston stuck within the brake caliper or brake pad wedged in the caliper slot

Effect

Noises
Uneven braking characteristics
Premature wear due to mechanical and thermal overload

Remedy

Disassemble the brake system and check it ✓
Clean the brake caliper and check for proper function of the individual components ✓
Check the brake caliper and pad for fitting accuracy ✓
Replace the brake pads and, if necessary, the accessory parts and brake discs ✓

Glazing of the friction lining

The friction lining has a hard, smooth, polished-looking surface that reflects incident light and is similar to a glass surface. This defect generally occurs on new brake pads with minimal time in service.

Cause

Overload of the brake pads in the bed-in phase
Repetitive high temperature change on the brake pad at short intervals
Long braking phases with low pedal pressure

Effect

Loss of friction on the brake pad, leading to poor braking function
Insufficient or one-sided braking function at the affected wheel brake
Noises (squeaking) during braking

Remedy

Replace the heavily glazed or damaged brake pads. Check the brake discs and replace them if necessary.

Brake pads with short time in service:

Grind the friction lining surface evenly ✓
Compare the thickness of all the brake pads and check for reusability ✓

Contamination of the friction lining surface

Brake pad has been contaminated by grease, oil or other foreign substances.

Cause

Deficient maintenance work
Defective drive shaft joints
Leak in hydraulic brake system

Effect

Insufficient or one-sided braking function
Braking causes noise

Remedy

Disassemble, check and clean the wheel brake of the affected axle
Determine and correct the cause of the contamination
Clean brake pads with sufficient thickness and grind the surface evenly
Replace heavily contaminated or worn brake pads
Complete the brake system and check for proper function

Failure modes for brake discs

Heat cracks

The friction ring of the brake disc has vertical cracks running all the way through from the edge to the disc chamber.

Cause

High alternating mechanical and thermal load
Overload of the brake disc in the bed-in phase due to force or sudden, hard braking
Use of the brake disc beyond the wear measurement
Brake discs of unknown origin with poor quality

Effect

Reduced braking function
Knocks at the steering wheel
Pulsing brake pedal

Remedy

Replacing the brake discs
Check the condition of brake pads and check for wear; replace if necessary

Always follow the maintenance and repair instructions from the respective vehicle and brake manufacturers in this regard!

Striations and grooves on the friction ring

The friction ring of the brake disc has horizontal striations or grooves on the contact surface.

Cause

Contamination or foreign bodies between the brake pad and disc
Brake pad of unknown origin with poor quality

Effect

Reduced braking function
Noises
Increased wear

Remedy

Clean the brake caliper and check for proper function
Replace the brake discs and brake pads.

Always follow the maintenance and repair instructions from the respective vehicle and brake manufacturers in this regard!

Different disc thickness/difference in thickness

The brake disc has a different disc thickness spread over its circumference. These deviations in geometry are also referred to as a difference in thickness. (DTV - Disc Thickness Variation).

Cause

Poor concentricity of the wheel hub or brake disc
Erroneous or improper installation of the brake disc on the wheel hub
Insufficient cleaning of the contact surface on the wheel hub
Long brake disc period of use with excessive axial runout

Effect

Wobbling motion of the brake disc
Axial runout with simultaneous low brake load
Brake judder

Remedy

Replace the brake discs and brake pads
Check the concentricity of the wheel hub and brake disc

Always follow the maintenance and repair instructions from the respective vehicle and brake manufacturers in this regard!

Subsurface corrosion of the contact surface

The disc chamber is corroded in places at the contact surface to the wheel hub.

Cause

Installation error
Insufficient cleaning of the contact surface on the wheel hub

Effect

Poor concentricity and axial run-out of the brake disc
Difference in thickness of the friction ring
Brake judder

Remedy

Clean the wheel hub and remove any rust
Clean the contact surfaces on the brake disc chamber and/or remove rust for an even surface
Replace heavily damaged or warped brake discs
Check the concentricity of the wheel hub and brake disc

Always follow the maintenance and repair instructions from the respective vehicle and brake manufacturers in this regard!

Brake damage assessment for wheel suspension and steering

As part of the brake repair, it is recommended that suspension components near the wheel brake, such as wheels, axle suspension and steering, be included in the check relating to the damage assessment. — Failure modes of wheel suspension and steering: Effect of defective components on braking characteristics

Influence of defective components on braking characteristics

Damaged or improperly function components in the wheel suspension or steering can affect not just the driving characteristics but the braking characteristics of the vehicle as well. If unusual noises, vibrations or unstable driving characteristics are detected during braking, the potential cause can stem from different vehicle components.