Uneven Brake Pad Wear

Apologies if I've posted this in the wrong section, since it's not , I think, a hydraulic issue.

I've just had to replace my front disks after only a year (about 4000 miles). When I took the calipers off I found three of the pads were fine, with plenty left on them, but the outside right hand one was down to the metal (hence the grinding noise that alerted me to the problem).

Any ideas what would make one pad bind? I noticed that when releasing the brake from stationary there's a slight creak. I don't think it's suspension noise from the back of the car shifting up/down, so it must be the brake. The front wheels were balanced recently and it's steady as a rock at high speed ie no steering shudder. I also don't think it's binding constantly - there's no notieable drag to the motion of the car.

I've searched the forum but haven't discovered a solution to this.

Matt

I think you will find that the caliper is not sliding across as it should do due to binding on the tiny tin "guide" on the bottom guide pin. Take the tin piece out and scrap it - unless you want to be a perfectioist and buy the whole caliper repair kit! I've never had resultant trouble after throwing it away.

Do you mean the clip which stops the securing pin from falling out? That was the only clip I recall seeing when I was lifting up the caliper.

Ah. I think you mean the clip which keeps the wear sensor in place?

While we're on it, which warning light comes on when the sensor wire wears down? The only warning I got was the sound of metal on metal.

Front discs should last around 60k - 70k miles on a manual, a bit more for an automatic. There is something radically wrong if they only last 4K. Check your calipers for sticking pistons and that calipers can slide freely on the posts. The pads
should wear a lot faster than the discs so its possible that the discs are rubbish.

Robert.

Im suprised you get that many miles from Xms front discs & pads. Our Xm has only done 19K in the 6 years we have had it, so i cant comment on the wear, although i have had front and backs replaced at around 75K miles, i expect they had been changed before this too.

Id definitely say autos are heavier on brakes than manuals, i feel i use the brakes a lot more in the Xm than i do in my manual daily car, you dont have the engine braking as you do when changing down in a manual.

I find pads and discs last around 40K miles, on my other cars. Of course we dont all drive the same, which might explain it.

Id also say a sticky caliper slider.

This post has been edited by Citroenmad on October 18, 2008 10:44 pm

Hi All

Heavy wear on one pad in an XM front caliper is regularly caused by wrongly fitting the pads with the pips. These pips must go piston side and if they are in the wrong way round the handbrake auto adjuster just screws one pad into the disk.

John

Pips were piston-side so no prob there.

Don't forget the old favourite of binding brakes caused by the brake valve not quite fully releasing either due to lack of clearance beteen the lever and the button or a partially seized lever. The bad news is that you have the hassle of removing and replacing the scuttle + end trims to gain access for checking.

Ken

Check that the caliper is free to slide laterally and that the rubber boots are intact.
Only the outside pad is affected which suggests that the piston is releasing the inner pad but the calliper is not releasing the pressure on the outer pad as it should.
Check also that the park cable is correctly routed through the wire loops and is not causing a pull bias on the caliper.
Presumably the need to replace the discs was caused by one being ruined by the pad metal contact. Discs last a little longer on automatics because they change down through the gears as the car slows, compared to clutch- brake.

Robert

A thousand apologies; I meant to write in my intitial post that I'd had to replace the PADS after 4000 miles, NOT the disks! Disks are fine (even if the one with the worn pad is a little scored).

I'm minded to agree with Robert and Techmanagain that because only one pad is binding, it's not a piston (or valve?) issue as they would surely cause both pads on a particular wheel to stick wouldn't they?

When I get a chance I'll take the wheel off again this week, remove the sensor wire guide clip and see if there's any stickiness in movement.

Robert, what are the rubber boots you're referring to?

Hi Reg...

Remove the retaining pin and pads. The caliper should slide freely on the caliper post. The small rubber boots are fitted between the fixed and sliding bits and enclose the caliper post to keep grease in and water/dirt out. If damaged, corrosion or gunge may cause seizure or stickiness. Hope that helps.


Robert

Hi All

On reassembly it is worth checking that one of the legs of the little "R" clip in the bottom locking pin does not foul the caliper. This can happen with a standard "R" clip which seem to have quite long legs.

John

Hi Matt.

Agree with all the forementioned, but also check the state of the brake cables. It's a dismally designed system and in time the cable outer case will fail at or about the last locating bracket before the caliper. This lets wet into the casing and seizure or stickiness may result. On my last one, the handbrake lever on the caliper had become prone to not properly releasing. Cured that by removing the rubber cover off the back of the caliper, clearing out all the congealed goo, 'working' it a bit and then re lubricating.
Totally agree with binning the warning cable clip on the bottom of the caliper; also check the pin itself isn't distorted or heavily corroded. Just to add a little to Xmexec's point, as well as ensuring the pip is on the piston side, you must also ensure the piston cutout clears it as you swing the housing down. To be fair, I'm not sure if you would manage to fit new pads with the pip on the raised part of the piston, but I've had one or two over time where it has been mis-fitted, leading to very lopsided pad wear.

Mike.