Car Mechanics (UK)

Strange brak­ing prob­lem

- Peter Clay­ton Iceland · Belgium · Belarus · Austria · SEAT León

I have a 2007 SEAT Leon Cupra 2.0 TFSI (240) with the stan­dard float­ing front calipers. Last year, when brak­ing, the car would jud­der, so I re­placed the front discs with Blue Print parts and EBC Red­stuff pads, which cured the prob­lem com­pletely.

How­ever, on the old discs, I no­ticed that the in­side face had two 4mm bands where the colour of the metal was much darker. The bands were lo­cated on the fric­tion area, right on the edge of the disc, and also on the fric­tion area fur­thest in. Hav­ing also in­spected the pads, I found that they were worn quite un­evenly both top and bot­tom, in line with the dark bands on the discs. If I’m not mis­taken this darker ma­te­rial is ce­men­tite, which hap­pens as a re­sult of over­heat­ing and con­tin­ues to get worse un­til the discs warp.

An­noy­ingly, only nine months and 3500 miles later, I no­ticed the in­side of the discs were beginning to show that same marks again. I as­sumed that it could be the calipers stick­ing, so I over­hauled the brakes from the pis­ton rings out. I re­placed the rings, cleaned the slid­ing pins, fit­ted new dust seals, applied fresh sil­i­cone grease and cleared off any rust. The calipers have never moved so freely, yet I be­lieve the bands are get­ting worse, although there’s no jud­der­ing yet.

What would cause these two bands to form on the in­side and only around the in­ner and outer perime­ter of the disc face?

On a side note, de­spite it be­ing a Cupra, I drive like a grand­dad and don’t brake harshly. Could my driv­ing style be the cul­prit here? After in­stalling the new discs and pads, I bed­ded in the brakes cor­rectly by slowly brak­ing for the first few miles, then do­ing pro­gres­sively harder brak­ing over time. I also did about six 60-20mph heavy brakes fol­lowed by a long cool-off pe­riod with­out com­ing to a com­plete stop, to al­low even cool­ing. Fi­nally, when re­plac­ing the discs, I used a run-out tool on the hubs and they are all within man­u­fac­turer’s tol­er­ances.

With­out see­ing the bands on the brake discs, it is quite dif­fi­cult to be cer­tain as to the cause. Ce­men­tite is formed at a tem­per­a­ture of around 650°C and, as a harder ma­te­rial, will wear at a slower rate than the sur­round­ing disc. If the lines around the disc are ce­men­tite, then un­for­tu­nately the dam­age is al­ready done (pos­si­bly by stick­ing calipers be­fore your over­haul) and the sit­u­a­tion can­not be re­versed.

Nor­mally, when brake discs have ce­men­tite, this is formed on the hottest part of the brake disc. As your ve­hi­cle has vented brake discs, I would ex­pect to see the ce­men­tite form in patches around the disc, be­tween the sec­tions of rib­bing through the cen­tre – that is to say, in the ar­eas which cool at the slow­est rate. Once formed, the ce­men­tite will be­gin to re­sult in DTV (disc thick­ness vari­a­tion), ac­cel­er­at­ing the con­di­tion and giv­ing a brake jud­der when brak­ing.

From you descriptio­n, you ap­pear to have cor­rectly in­stalled and bed­ded in the new brake discs, tak­ing all the pre­cau­tion­ary mea­sures re­quired. The Red­stuff brake pads you fit­ted have a high ce­ramic con­tent, which is one of the rea­sons that the dust level is low. One of the dis­ad­van­tages of ce­ramic brake pads over the con­ven­tional ma­te­rial is that they do cause more wear to the brake disc.

As your lines seem to be con­tin­u­ous, this may just be a wear char­ac­ter­is­tic. Given your driv­ing style, you may find that using ei­ther the Green­stuff op­tion or gen­uine VAG brake pads of­fer a more ap­pro­pri­ate op­tion, although you would have to con­tend with more brake dust and a faster wear rate on the pads. This may be prefer­able to a higher wear rate on the brake discs.

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