Inner and outer brake pads on one side wearing out much faster than brake pads on opposite side.
The list of possible causes for this pad wear condition is much longer than what most technicians think. Approaching this problem with an incomplete list will result in fixing the symptom not the problem in many cases. The list of possible causes include:
- Sticking caliper piston on side with problem
- Check valve brake hose on side with problem
- Restriction in ABS modulator preventing fluid release on circuit with wear problem
- Plugged or restricted vent port on circuit with wear problem
- Opposite side brake hose with restriction
- Air in opposite hydraulic circuit from problem (i.e. left pads wearing, air is in RF/LR circuit)
Accurate diagnosis of this problem involves 2 components. The first is approaching the problem with a complete list. The second is applying a logical approach to the diagnosis. The first component is listed above. The second component involves the following steps:
- Check brake pedal height. Judge pedal against similar vehicles. If pedal has excessive travel or feels spongy bleed and flush entire system paying close attention to the circuit opposite the side with the pad wear.
- Test drive vehicle to bring brakes to operating temperature. Raise the vehicle on a lift and check for wheel drag on all four wheels paying close attention to the side where the pad wear is taking place.
NOTE: If the vehicle cannot be driven due to metal to metal wear the system will have to be restored to a functioning state before complete diagnosis can take place.
- If vehicle pulled to side with problem during test drive opposite side brake hose may be restricted. If no other causes of the pull can be determined then brake hose replacement should be suggested.
NOTE: Many FWD vehicles will NOT pull with a side to side brake imbalance due to what is known as negative scrub radius (See Figure 11.2). If the vehicle does NOT pull, it does not guarantee the opposite side hose is not restricted. If pressure gauges or clamping plate gauges are available, they should be used to determine side to side braking balance. If not available, a good practice is to perform all other repairs and instruct customer to return in the miles it took to wear the brake pads out for a checkup. Inform them that the opposite brake hose may still be an issue and this step will determine if it is necessary.
Visit us next week as we discuss stoplight switch adjustment and pad wear. If you have missed any of our other Tech Trick Tuesday Blog Posts, you can read about them on our 101 Brake System Tips page.