987.2, rusty discs and the MoT man?

[911hillclimber]

MoT due on my 2009 987/2.9 and getting lots done that are needed, exhaust bolts and such like.

Last year the MoT man (used the same village garage for 30 years) questioned the rust on the front discs even though the efficiency test was perfect and bery balanced across the front. The rears were rust - free!

Despite leaning on the brakes the front inner surfaces are not clean, the outer surfaces are great, so I'm expecting some difficulty.

I am tempted to replace the perfectly serviceable discs/pads for new a few days before the MoT, BUT, if the brakes work can he fail the brakes simply because he feels the corrosion is too much?

Any experiences please?
Graham.

 

[hotfrog1]

MoT due on my 2009 987/2.9 and getting lots done that are needed, exhaust bolts and such like.

Last year the MoT man (used the same village garage for 30 years) questioned the rust on the front discs even though the efficiency test was perfect and bery balanced across the front. The rears were rust - free!

Despite leaning on the brakes the front inner surfaces are not clean, the outer surfaces are great, so I'm expecting some difficulty.

I am tempted to replace the perfectly serviceable discs/pads for new a few days before the MoT, BUT, if the brakes work can he fail the brakes simply because he feels the corrosion is too much?

Any experiences please?
Graham.

This may be useful: https://www.motester.co.uk/mot-test-of-brakes/

 

[911hillclimber]

Thank you for that link, interesting!
if they work and are not worn or damaged, all is ok.

 

[dpoynton]

Would "skimming" the discs give them a longer life or has this been done before?

David

 

[Gordon Attar]

They need to pass the brake stopping test whether they have rust on them or not and it can take 6 to 10 hard braking stops to remove sufficient rust.

Would recommend that you put in for the test and only change them if they fail.

 

[911hillclimber]

Thanks all, It will be irritating to have to replace perfectly good brakes for new on the opinion of the MoT man, the front parts are about £300.
my '73 911 does not have rusty discs, so why do the modern brakes rust?

 

[Gordon Attar]

No lead in the brake discs anymore

Any chance your 73 car is a garage queen(quite rightly) and only gets wet when you wash it and you probably dry the brakes with a short drive afterwards.

 

[Gordon Attar]

No lead in the brake discs anymore

Any chance your 73 car is a garage queen(quite rightly) and only gets wet when you wash it and you probably dry the brakes with a short drive afterwards.

 

[911hillclimber]

Yes, it gets pampered, high days and sunny weekends, only 2000 miles a year.
but my 2014 Skoda daily does not have this issue.
MoT man said he sees the rust issue on the inside face regularly.

skimming...does anyone do that any more?

 

[vitesse]

I can't see there being lead in cast iron as a constituent-it's melting point is too low.More likely that the carbon( which is what reduces rusting in cast iron-compared to steel) content in the iron has been reduced when using recent technology.

 

[hotfrog1]

I can't see there being lead in cast iron as a constituent-it's melting point is too low.More likely that the carbon( which is what reduces rusting in cast iron-compared to steel) content in the iron has been reduced when using recent technology.

I think it varies simply on the spec of the disc material. We have a Discovery Sport (2017) and the rear brakes rusted like they were soluble. In well under a year the rear brakes rusted so badly in 9K miles they had to be replaced. They had patch rusting that caused thudding under braking. The front discs are fine and still like new! We have a 911SC and the brakes rusted on that quite badly when it was used a lot but it sees few miles now so difficult to form an opinion about that. Our 986 Boxster S discs do rust. In 18y and 65K miles it it now on its fourth set all round. The current ones have been on for about three years and are fine but it is a garage queen and has always been so. We always make sure the brakes are dry before it is put away.

Why not make them from stainless iron? Probably cost but could be an option or is there a technical reason? Lotus Elise used to have alloy discs but they dropped that idea in about 2000.

JZ Machtech have a machine that can skim discs in situ. I remember being shown it at an open day.

 

[911hillclimber]

MoT today for my 987.2
passed with a verbal suggestion, but not about the brakes.
before the session I took the car to some quite roads and caned the brakes so the abs came on repeatedly so polishing the discs (?)
anyway, the brakes passed no issues and no comments.

Issue to fix is the non working head lamp washers, possibly a legacy of the bumper-off for the AC radiator change.
I hope it is just a crimped tube...

 

[legin]

The reason modern discs corrode more is they are a lower grade of cast iron these days. Cynical way to extract cash ! I have a mitsubishi shogun and the original discs are still going after 15 yrs and a VW with terminal discs after 3 years.

Quality discs like AP and Alcon resist better. Discs wouldnt be made from steel including stainless steel as its both more expensive but most importantly not as dimensionally stable especially when heated.