Car Mechanics (UK)
Strange braking problem
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.