Rear disc wear on 2000 2.7 with 50k

[timbo]

Hi All, thought you might be interested in some pics of my rear discs on my 2000 2.7 I've just replaced after 50k. I have seen posts about similar wear on front discs but rears. From outside the car you wouldn't know there was a problem, it was only when the car was up on ramps that this was spotted.

Incidentaly, replacement was a very simple job the only additional tool needed was a 10mm hex socket.

(think I've managed to upload pics! here goes anyway.)

[Westcoaster]

So from the picture, the left rotor, it would appear that the pad was only making partial contact with the rotor? What did the pad look like?

[timbo]

Very uneven and cracked, despite this the pedal was good. Don't yet know much this will affect braking as I haven't had time to bed them in. This is my first Boxster and have felt that quite a bit more effort is needed on the pedal than other cars I have driven, but I read here thats normal.

[rsfeller]

My rears (original) look amost like yours. I feel the calipers do drag a bit but wear looks fairly even from what I can see. I'm going to rebuild the calipers and get new pads/rotors next spring after getting a few other projects done. Braking is fine but you do here the scraping of rust excessively at first after sitting a week.

If I took a photo mine would look just like yours.

[Westcoaster]

Very uneven and cracked, despite this the pedal was good. Don't yet know much this will affect braking as I haven't had time to bed them in. This is my first Boxster and have felt that quite a bit more effort is needed on the pedal than other cars I have driven, but I read here thats normal.

Interesting, both pad surfaces should be the same, full contact. In this case there would appear to be 'something' out of alignment.

[RFM]

For those who are interested, you can avoid this problem if you keep the calipers inside clean after a pad change so that the pads can slide easily, bleed the brakes regularly. Start bleeding the wheel which is furthest from the master brake cylinder in this order for LHD cars = 1/ right rear wheel - inside nipple first, than outside nipple 2/ left rear wheel - inside nipple first, than outside nipple 3/ right front wheel - same method as rear wheels 4/ left front wheel - same method as rear wheels. Hope it helps.

[timbo]

Not sure of the cause but its definately nothing to do with alingment, uneven pad wear is caused by the disc surface itself not being even. As for cleaning and bleeding, well all the pistons in the calipers were free and working fine, I don't think you would get this kind of identical wear pattern on both rear discs, that would mean there would have to be an identical problem on both rear calipers.

Its been said befor that the cause might be due to moisture, the outside surface getting more air to dry the surface, not sure about this but when it happens its usually the inner surface that seems to corrode the most.

Edited August 31, 2008 by timbo

[RFM]

Brakes are cooled from the inside to the outside ( that's way the wheels become dirty with brake dust ), so more air to dry on the outside as you said makes no sense IMHO.

[timbo]

I'm not talking about cooling, I'm talking about drying, It stands to reason that the air under the car when parked overnight is likely to be more humid therefore allowing corrosion. Better ventilation to the outer surface would mean that the disc will be dryer, makes perfect sense to me. I notice a number of other components under the car also suffer from corrosion in this area. It may be that as air leaves through the wheels salt and other crud gets deposited on the discs.

Can you explain how bleeding the brakes helps prevent this.

Edited August 31, 2008 by timbo

[RFM]

If the calipers are not bleeding properly and in order as i mentioned in my earlier post, the risque of a small air bubble is existing in the inside part of the caliper, which means that the brake pressure is slightly minor in the inside part than in the outside part. Why should some cars have this issue of inside corroded rotors and others not, there is water and dirt in most places over the world. On a perfect working system the pads will clean the rotors on the same way inside or outside. Drive you car on a brake test bench before and after the bleeding is executed, and note the difference in efficiency between both tests from slightly braking till full braking. Forum member Westcoaster is not so far away with his theory of "something out of alignment" the result is the practically the same.

[Westcoaster]

Thanks for the vote of confidence RFM!

Disc brake systems are designed to allow all contact surfaces to come together with even pressure, the pads should contact the rotors evenly across their entire surface. To have this system work correctly the calipers and the piston(s) have to move freely as brake pressure is applied and they clamp onto the rotor and self align to the friction surfaces.

My comment about alignment was in reference to this.

timbo: you said that the pad was

Very uneven and cracked

I would have expected that there would either be friction material missing to correspond to the area of the rotor that appears untouched or that the backing plate for the pad was not flat against it's mount.

As for moisture IMHO, a few minutes of brake use should eliminate all traces of moisture from the pads. I still beleive that uneven wear like that is the sign that either the system was re-assembled incorrectly, something is broken (jammed or seized, either a piston or slide) or the brake pad is defective and the material is breaking away.

Edited August 31, 2008 by Westcoaster

[Tool Pants]

It looks like those rotors have started to rust out, and it does so from the rear of the rotor. The front of the rotor always looks fine. Only on the UK message boards have I seen pictures like that. Must be all the rain and/or the crap they put on the roads for winter.

Pictures from PGrimston on the Porsche Club Great Britain board.

[timbo]

Nothing broken or misaligned, as I said how would you get the same pattern of wear on both rear discs otherwise, and it seems its quite a common Boxster issue. As for bleeding and air bubbles, check your physics, there is no way more pressure can be generated on one piston unless of course its siezed, and as I said mine aren't.

Thats interesting Toolpants, same type of wear as mine.

Edited August 31, 2008 by timbo

[RFM]

I did what you mentioned about physics, air is compressible hydraulic not. Anyway it's your car.

[timbo]

Yes air is compressable thats my point, check out Boyle's Law, the pressure acting on a bubble of air in the system will cause a universal change in pressure as it is compressed, you simply cant get more pressure in one area.

Anyway, I've just looked at the Brembo brakes site, they show pictures of worn discs like mine, they call it 'Vitrification' apparently caused by corrosion, how it happens on so many Boxsters on the inside of the discs....I don't know.

[Westcoaster]

Timbo, you've got me (regarding alignment) since both discs exhibit the same problem!

When you say "so common on Boxster's" I wonder how often it is seen over here, I live in Victoria which has a similar climate to England and haven't see that before on the dozen or so brake jobs that I have done (not that I have done many Boxsters).

It is interesting that the pictures that Tool Pants posted all came from the UK!?

Did the Brembo site mention that certain cars were prone to this or that certain environmental condition were at play?

[timbo]

Hi Westcoaster, well I've noticed this problem crop up a number of times on the various Boxster sites, can't remember if they were UK based though. I have seen the same problem on other cars, I recently replaced the rotors (both fronts) on my daughters Renault Clio it had a similar wear pattern, they have a single piston floating caliper set up so less likely to be related to sticking pistons. Perhaps it is due to our climate coupled with the amount of salt they put on the roads in winter. No mention on the Bremo site of this being specific to particular makes of car. They did show pictures of damage resulting from mis-aligned calipers but thats not the case here. I'm fairly sure its due to corrosion, though how and why I'm not sure. I'm thinking that some kind of pitting in the disc surface starts the process off, these pits gradually getting bigger as time goes on until large areas of corrosion appear, who knows. Thanks for the imput.

[Beelzy]

I tend to lean with the caliper bleed theory. That or warping rear calipers due to heat buildup.

Improper bleeading is notorious for causing that sort of disc wear on GM vehicles even with single piston calipers.

If seemingly a problem for UK cars, maybe the same guy is doing all the brake jobs there. :D

[timbo]

As I pointed out in an earlier post its not possible to have different pressures on the pistons. OK yes pistons can seize but not in this case. Do you think calipers can warp? or do you mean discs? How do you account both discs needing replacement, and the corrosion in the same place on each disc. This would mean there would have to be an identical problem on each caliper on each pair of pistons on all the cars that do this, seems unlikely.

When I was working for a garage back in the 70's we never saw this problem.