[Non Corsa] Uneven Disc Wear - What's Caused This?

[Hardcore]

Started to get a funny noise from my front OS wheel earlier in the week.
Pulled the wheel off and found the disc like this on the inside only. Outside is fine




What could have caused this to happen? I thought if the pad covers almost all of the disc that as soon as the piston pushes it, then it pushes all of the pad into the disc as well.
Strange as it is they still worked really good just before i discovered the state of them and stopped driving it.

[tanktop]

Eventhough the contact of the pad on the disc looks fine looking at that I would say this isn't the real case. I've had the same on a motorbike before and the pistons were coming out cock eyed.

I would remove a pad and support the caliper, gently push down on the brake pedal then check the piston keep doing this until it is 3/4 of the way out, then spray the whole thing with brake cleaner then use a piece of soft wood and a G clamp and slowly pushthe piston back in fully.

Repeat at least 4 times spraying each time when 3/4 the way out you should find the 3 and 4th time the piston pushes in very easy if so that should of sorted the problem out.

If not then gently lift up a small part of the dust boot and spray the cleaner in there making sure the gap you make allows any bits to be cleaned out and repeat the above.

[Rob]

rocksta covered it but id agree, the pistons partially seized. sadly that disk and pads are shagged. upgrade time tam..... wonder if passat 3bg 312mm disks and tt calipers fit?

[Hardcore]

Cheers guys

I've got TT calipers, carriers pads an discs ready to go on shortly. Just waiting on new bleed nipples to arrive and some ronseal silver to clean them up before fitted.
I'm going to try and clean up these carriers though as well whilst they're off in case i put it back to standard when selling.
Will probably put myself through the windscreen after changing them with the difference compared to these.

[Taurus]

It's something that happens when the pad lugs and caliper corrode, they stop sliding properly. So when the piston presses the pad the pad doesn't slide evenly, the part nearest the lugs doesn't move, only the part further away. You can see that from the pattern on the disc - the outer section, nearest the caliper carrier, is where the pad isn't making proper contact, whereas the inner section of the disc is making contact. Effectively the pads aren't sliding in the carrier - so only the edges of the pads are able to be pushed into contact.

It's why you need to strip, clean and slightly lube the lugs and carrier regularly. Toyota use a polished metal clip where the lugs slide in the carrier. It's only wafer thin but it does the job. It's a cleverer design than the Vauxhall system and avoid this problem developing.