Brake disc corrosion

[Turbocharger]

OK, it's my MG and not my LR, but I'll be driving one full-time if I can't get the other back on the road, so there's LR content there...

The parking brake has been the bane of my relationship with Mr MoT since I got the car. The mechanism in the calipers is known for filling with water and seizing, so the caliper was barely moving, little or no brake effort and the disc went rusty around the outer edge, in line with the pad guides (coincidence?).

2 years ago I unseized the caliper, rebuilt the park mechanism and replaced the discs and pads. It passed the MOT but the park mechanism went the same way again, and 12 months later the new discs had surface rust around the outer edge.

12 months ago I replaced the calipers with new units, replaced the pads and scuffed the surface rust off the discs. Again, the disc has rusted around the outer edge, so I put new pads in it and crossed my fingers for the annual test. It's failed the MOT today with low park brake effort (12%) on both rears.

The discs have been replaced, but they're a symptom and not the problem. If I replace them again, they'll go the same way unless I can find the fault.

The calipers have been replaced, and they're doing the same thing.

The pads have been replaced (again!) - no joy.

The pads are free-moving in the caliper (they fell out easy enough) and the slider pads are bright and smooth.

The whole thing was assembled with copper grease on the backs of the pads and the 'shoulders' where they move in the caliper.

All the parts are Mintex or Ferodo, no Chinese copies here.

Help me stop tearing my hair out?!? I'll post a photo shortly.

[Bowie69]

Had this on a 1995 Accord, I found the fault, I am pretty sure... it was the handbrake cable outers compressing due to old age reducing pressure on the actual pads -it sounds like a similar design caliper arrangement, so would be worth checking. I could see the cable outers shortening significantly as the handbrake was applied, then you ran out of lever eventually.

I didn't ever repair it as I sold the car It did work, just not quite to Mr MOT standards.

[Turbocharger]

Thanks, but the cables are the one bit that's working well - there's full movement on the park brake levers (on the back of the caliper) and it "feels" right on the lever - which it certainly didn't when the mech was seized!

Here's some pictures of the offending parts:

No idea why the brake pad lining has fractured away either - is this a sign of overheating?

[Range Rover Blues]

I can see what you mean about the corrosion now. I have come across similar on other cars and I think it starts with a lack of use, my RRC does it because the disks don't get enough use to keep them clean.

Because it's the outer edge of the disk that will effect the braking power available, it reduces the mean radius of the braking surface, plus the rust cuts the pad material away until the disk forms a scale which you can often see as a dark grey area.

This won't be helping the handbrake at all, whether it's making that much difference I don't know.

[simonr]

How about a sacrificial anode to corrode preferentially to the disk?

You need to find a lump of Magnesium and give it a good electrical connection to the thing you want to protect.

They work pretty well.

Si

[Range Rover Blues]

I bought such a kit from the bay last year (it's still in the boot) to protect my LSE. No reason it wouldn't work on the disk I guess.

[disco_al]

the rear discs on my disco were like that when i changed them last year. what condition are the sliders in? could be worth giving them a good clean, and make sure they are well lubed up. if they are sticking slightly, or even just binding, then when you apply the brakes it causes the caliper bracket to twist slightly.

[Turbocharger]

The sliders are ok (free, smooth and they were changed with the calipers anyway), and they're not going rusty through under-use - I do 20,000 miles a year in the car! Somehow the pad isn't meeting the disc at the outside edge, and I don't see why since I've replaced the whole thing, piecemeal. In the same fashion I think magnesium would address the symptom rather than the problem - any surface corrosion should be swept away as it is on the inner radii, the front discs, and every other car on the road.

There's something wrong with it, and it worries me (and it's failed the MOT).

[Bowie69]

OK...... re-read a couple of times now.....Rust is much harder than steel, as such it will not wear at the same rate as the rest of the disc, this means even if you clean rust off a ruefully, you'll still get premature pad wear, and the rust will reappear.Rusted discs were a symptom, but they are now the problem, hopefully you now have it fixed

[Retroanaconda]

I found that some 'enthusiastic braking', sort of 60mph to 10mph and repeat, cleared some odd corrosion patterns on my rear disks on my 90. Just make sure you allow some time for them to cool down afterwards on the way home.