Uneven Brake Pad Wear

[elebub]

In the process of refurbishing the front brakes on a BMW E34 530i, new pads piston seals and rotors etc. With reference to the pictures, can any one give me a clue as to why this has happened to the old offside rotors? The front face is normal but the inside face (piston side) is not and the pads were nearly down to the metal, whereas the nearside pair (brake pad sensor side!) had a healthy amount left. Is it just a case of worn piston seals?

 

[shenion]

Check the pins the calipers ride on. They should move freely.

Also looks like the inside pads are smaller.

 

[Wozzaaah]

I'd guess the caliper on the 'unworn' side isn't sliding properly. Remove the sliders, inspect for pitting and just general wear, if they look ok, polish them up, clean up the bores that the sliders fit in, lubricate the sliders with copper grease and refit the sliders.
Normally the rubber gaiters on the sliders are part of the calipers and can't be replaced on their own so if they're split then unfortunately, they'll allow water in.

 

[elebub]

Check the pins the calipers ride on. They should move freely.

Also looks like the inside pads are smaller.

Thanks shenion, I will make sure I put copper grease on the guide tubes when I reassemble the units.

Both pads are the same size but the inside one was only making partial contact with the rotor.

 

[shenion]

It is more than just making sure that are greased. They can get pitted, bent, or just wear. I have seen them with a notch in them.

Did the pads wear flat across? Usually, if they wear at an angle, it is a sign that one pin is bad.

 

[Wozzaaah]

Sounds like the sliders were almost seized. Did the car pull to one side under braking?
Not sure if you can buy the sliders separately but if not and the car's a keeper then if it happens again I'd say invest in a new caliper(s).
They'll set you back a few quid but at least you know the job's a good'un.

 

[Seano]

looks like partially seized piston, if double sided, or slider pins sticking in the guides, my wife's mr2 turbo is notorious for this, literally have to strip cals down every 6 mths to redo them and keep em sliding freely, HTH

Sean.

 

[pedrobedro]

Slider sticking is the problem, the rubber spring pushes the opposite side to the pistons to open them up and when it sticks only the piston side releases. Copper slip dries out and makes the problem worse, get some white brake grease which is also compatible with the rubbers.

 

[elebub]

Thought a diagram might help. The excessively worn area is in red. Checked the guide bolts (no 8 in the picture); not excessively grotty and polished up OK with no lumps or bumps and not bent. Rubber bushes are in good condition. All looks OK with no excessive wear anywhere. Weird.

 

[Wozzaaah]

Regardless of what they looked like, did the sliders move freely as you removed them or did you have to knock them out? They should slide easily just with finger pressure.
Take the good side apart and see how freely they move to get an indication of what it should feel like.
As we've all agreed, it has to be the caliper not sliding correctly, if all was working ok (or if it was the piston causing the problem) then the wear would be spread evenly between the two pads.

 

[elebub]

Wozzaaah
Slide out easy with finger pressure. Stripped both calipers down last week to refurbish and thought that the cause would become blindingly obvious (not).

 

[KTMMICK]

I would change the discs for sure.........and are the pistons damaged/ corroded at all?
Strip it all down..........dont dismantle the caliper completely, but push the piston out carefully as far as you can to check.........good luck! If you need parts German and Swedish will be your best bet.

 

[CKJDJK]

Another cause is often the pad sticking in the carrierand being unable to come off the disc properly. This causes the pad to wear unevenly and the disc to start rusting. The rust wears the pad, usually on the outer edges of the pad quicker than the pad area that runs on the clean part of the disc. The rust continues to build up on the disc eventually coming off in flakes causing vibration and rapid pad wear. If caught early enough you can use a flap wheel on an angle grinder to clean the disc while spinning the disc by hand to keep it even.
Frequent dissmantleing and cleaning of the Pad carrier, Item 3 in your drawing, particularly the area in contact with the pad metal backing will help. A bit of copper slip here helps also. The pad should be able to move freely but not be too loose to cause rattle. Some cars have a stainless steel slipper at this area in which case remove it and clean the area underneath as well as the slipper. The pad in this case will be a little stiffer than the older type but should still be able to slide.
As already said by others, the slide pins need to be kept clean and greased to enable the caliper to slide correctly.
Chris

 

[reno]

I've suffered with a very similar problem before and found that it was the pads binding in their guides rather than the carrier assembly sticking on the slider. This causes the pad to pivot about the guide when you press the brake pedal rather than slide along it which is why the corrosion is on the outside edge of the rotor.

You may find that the guide is caked with baked on brake dust a good clean out with brake cleaner, a small wire brush and a needle file should clean it out.

Out of interest has the car been left stnading for a while as thats when I find it occurs.

Hope that helps.

cheers, reno

 

[Add]

I've suffered with a very similar problem before and found that it was the pads binding in their guides rather than the carrier assembly sticking on the slider. This causes the pad to pivot about the guide when you press the brake pedal rather than slide along it which is why the corrosion is on the outside edge of the rotor.

You may find that the guide is caked with baked on brake dust a good clean out with brake cleaner, a small wire brush and a needle file should clean it out.

Out of interest has the car been left stnading for a while as thats when I find it occurs.

Hope that helps.

cheers, reno

Yeah what he said

Every BMW I did brakes on had the caliper carrier / pad guides caked with crap.
I used to take ages to do pads but whats an extra 5 minites a side too save this happening to the brakes.
We used to see wear like this every now and then

 

[elebub]

Thanks for all your replies. The old piston side pad did have a more than generous helping of Brake Pad Paste and Dust Casserole which cannot have helped things. Relieved that it seems to be a clean up job rather than structural. New piston seals and new rotors going back on and a note to clean out the dust on the pad carrier on a regular basis.