[OT] Car brakes

[Aaron]

Hello and sorry for the possibly OT post.

I have a 1995 (M) Vauxhall Corsa Merrit (1.2) and have been experiencing some problems with the brakes, specifically the front right. My dad used to be a mechanic and can help me with this so I'm not going into this completely blind!

When we got the car a few years ago it required new pads so we fitted them. We found the caliper was of the sliding type (standardish on new Vauxhalls so I understand) and 'slides' on a axel with a screw thread on on one end. Being Vauxhall it needs a 7mm hex key (a size not usually found in a tool kit), someone had damaged the caliper bolt having taken a pair of mole-grips to it instead of using the correct hex piece.

We replaced the pads/springs anyway and put it back together, a few months ago the front right started making a grinding noise and upon further inspection a little while later we found the pad had cut a groove into the disks. We replaced the pads and disks (both sides) as explained in the Haynes manual... It still made a grinding noise so we thought it could be the damaged caliper guide bolts (not allowing the caliper to fully 'open').

We aquired and fitted new bolts but alas it's still grinding and upon further inspectin we caan see the brake pad is 'toeing in' on one side to the disk for no apparent reason. The noise seems to happen once per rotation... Given theses are new disks we are guessing they shouldn't be warped and were hoping some other people may have some ideas?

Thanks in advance.

P.S. The grinding noise could also be described as a rubbing/binding noise.

[Colin Wilson]

Try uk.rec.cars.maintenance

[Andy Dingley]

Then whatever else is wrong with the pads & calipers, your disks are warped. This can happen by poor fitting, by over-torquing wheel bolts, by poor run-in, or simply by buying poor quality disks.

I wouldn't want to work on disks without a dial gauge and magnetic stand (not too expensive from Draper or eBay). Although they usually do go together without problems, you ought to check runout anyway.

What's the pad wear like ? The usual indication of caliper slide problems is when either one pad wears before the other, or when they wear tapered.

7mm Allens are easily available - Sykes-Pickavant and people do them, specifically labelled as brake caliper wrenches - I think it was Ford who started using them for this first. I have one I bought 15 years ago and I think that's the only thing I've ever had cause to use it on.

[Aaron]

It could be a combination of all of these factors :I.

We have these I think but whether they were used or not is another matter (I didn't do them), the most I ever done on a car is the electronics.

I may take the wheel off tommorow and have a look.

Indeed, a quick trip to Halfords got us one.

[andrewpreece]

One possible explanation for 'toeing in' of a brake pad ( i.e. it wears asymmetrically and develops a taper from one side to another ) is that you have dual brake pistons/cylinders for each brake caliper, and one of them is siezed. Typically, the seized piston will move the brake pad against the brake disk when the brake is applied, but it will not retract sufficiently again, even the tiny amount that is needed to free it from the disk, when you remove your foot from the brake.

The end resut is premature wear on one end of the pad, leading to a tapered appearance, usually accompanied with lots of brake dust coating your alloys, increased petrol consumption, and a squeak coming from that wheel, best heard with the window down.

The symptoms you have described are slightly different to this explanation as you mention a grinding noise and a groove cut into the disk ( possible if the taper on the brake pad becomes severe, I suppose ). Anyway, that's my theory on what might be happening. To fix it you need to undo the calipers from the wheel, then using a piece of wood to equalise the brake pistons' movement, apply the brake pedal until the pistons pop out ( under no circumstances remove the brake caliper completely from the car then attempt to prise the pistons out as they will probably be in very tight - let the hydraulics do it for you ). Then clean/replace pistons/seals as appropriate.

Andy.

[Aaron]

Sounds plausable and I think I remebered my dad mumbling about cleaning the piston and re-doing the seals. Maybe we'll give it a go.

Thanks.

[Dave Plowman (News)]

A standard allen key is unlikely to be long enough anyway to undo the pin. Eurocarparts sell the correct tool for a couple of quid - other cars use the same brakes.

But it's better to buy a set to fit a 3/8 drive socket set - Halfords do them. That will also give you the one for the discs, as well as other things round the car.

[Dave Plowman (News)]

I've never found the need. You have to make sure the mating surfaces are clean on assembly, though. And I'm not convinced that uneven tightening could cause a solid flat lump bearing against another solid flat lump to warp. Or at least not before the wheel fell off. ;-)

IMHO, the likely cause of warping is stopping fast from high speed and keeping the brakes applied when you stop, causing uneven cooling.

Think I first saw this on a BMW.

[Dave Plowman (News)]

I'm sure they exist, but my largish BMW has single pistons both front and rear. Makes for a very simple system. My old SD1 has four per caliper. ;-)

[Andy Dingley]

Although they usually have an average of 3 1/2 in service at any one time.

[Dave Plowman (News)]

Heh heh. Rebuilt them recently with 'new' pistons and seals. Although the pistons weren't new, but reclaimed from several ordinary four pot types - mine has the twin circuit vented discs, so the calipers are very expensive on exchange. It seems very strange to me the pistons are chrome plated for such an arduous position - especially since there's no dust cover. But I got a seal kit from my local factors, and the bores were fine.

The main problem with the SD1 brakes is those rear drum self adjusters which have a mind of their own.

[Aaron]

This is what we have, we used such a tool to remove the bolts and we discovered the damage on them.

[Rob Morley]

Why? Chrome is hard and corrosion resistant.

[mrcheerful]

As soon as a stone hits, the chrome chips and the steel below rusts (likewise if the ground to size chrome has a speck of steel showing), if the pistons were good stainless then they would never need replacement.

mrcheerful

[Dave Plowman (News)]

But they had all corroded which is why they needed replacing. ;-)

[Andy Dingley]

Resistant, not rust proof. Look at any of these calipers and you're guaranteed at least one piston with rust pitting through the chrome. Road salt is a harsh environment.

Stainless pistons are much more rust resistant, however they're softer and more likely to develop scoring.