Brake pad wear problems

[Crockett]

We recently had cause to replace a wheel bearing on my son's car. During that time I took a look at the brake disc and remarked that it may well have been replaced at some point, as it didn't look too badly worn at all. Both brake pads looked to have about 50% life left in them. Everything seemed to go fine with the bearing replacement but then the bad news started to come.

We noticed a slight scraping noise from the brakes, not continually at first. Usually applying the brake and then releasing them would cause it to stop. Checking the bearing revealed that all still seemed to be okay (no wobble) but the noise was still there at times and was perhaps becoming more continuous.

Due to time constraints on his part, it got put on the list of "things to look at" but not soon enough it seems. The scraping then got worse still (over a couple of weeks) and on taking a look at the brakes yesterday, the outside pad has basically gone and the disc has been badly scored by metal on metal contact. The wheel was hot so clearly the pad had been binding on the disc for some reason. The other (inside) pad still has 50% life and that side of the disc is also still smooth.

Anyone have any ideas what may have happened to cause this? Something perhaps dislodged and caused the pad to always touch the disc? The relative position of the hub (with the new bearing) has shifted to be closer to the outer pad (but wouldn't the calipers self-adjust to allow for that? Perhaps not if it's too much, due to the inner bearing not being properly seated or the caliper being too rusted to retract fully? I don't know.)?

Any help would be appreciated. A new disc and pads will be needed for a start but I don't want the same to happen again.

Cheers
Daryl.

[Dizzy]

Daryl,

Could it be a partially-seized caliper piston?  This can happen if the weather seal has become displaced (usually by clumsy fitting).  This could have happened years ago but no problems until corrosion sets in and the piston resists retraction.

The usual sign of a seized caliper piston is a continuous scraping noise a few miles into a journey.  The pad is lightly rubbing on the disc at the start of the journey because it can't fully retract.  The first few miles heat up the piston, brake pad and disc and the expansion presses the pad tighter onto the disc.

In your case you know which corner has the problem (whatever that might be) but for the benefit of anyone else who experiences a scraping noise that might be the brakes and the source is unclear, feel the wheels and the one that's hotter than the others will be the one with the binding brake.

If it is corrosion on the piston, there will probably be pitting.  However a certain amount of pitting can be tolerated.

Quite possibly the cause is something else altogether, but the above just might help.

[Dizzy]

Another thought...

If it is a seized or partially-seized piston, and you need to extract it from the caliper, do so by using the car's hydraulic system before you remove the caliper.  An airline will do the job, given enough pressure, but it's easier and safer under the control of the brake pedal.

[Jason]

Yes be careful with an airline otherwise your piston could go flying off into oblivion or through a window :-( By far better to use the hydraulics to pump it out as far as it will go :-)

[Dizzy]

Jason wrote:

Yes be careful with an airline otherwise your piston could go flying off into oblivion or through a window :-( By far better to use the hydraulics to pump it out as far as it will go :-)

It could be worse than that even.  Several years ago I lost about 8mm of a finger tip using an airline for this.  The piston wouldn't budge so I kept turning up the pressure.  At about 80 psi shot the piston out but my finger tip was between it and the caliper!

The finger tip was hanging by a bit of skin so I held it back on and got my wife to fix it in place with some plaster strips.   Should have gone to the hospital but that was several miles away.  To my great surprise, the 'graft' took hold.  Weeks of pain though, presumably because of nerve damage, so a lesson never forgotten!

[Jason]

Ouch! At least I only had to pay for the window (not my workshop) and suffered endless mickey taking :-)