silent running

I just changed my discs as they looked low and wondered if anyone can shed any light on what I found out? It's the 294mm 4 pots on a classic.

The old discs were down to 20.5mm so obviously no good any more and they had a fair old lip on them. but the weird thing was they were unevenly worn. Both discs were noticeably thinner on the inside surface than the outside, which seems odd considering there are two equal sized pistons each side of the caliper. Also, the pads seemed much more worn in the middle than the ends and more worn towards the 'bottom' than the 'top' of the pad.

After a bit of a struggle getting the replacement discs in (2nd hand with 23mm on them), I finally managed it and took it for a test drive. Straight away it didn't feel right, as if the brakes were stuck on, and sure enough after a bit of a run, the nearside started smoking a little. I got out and sure enough it was hot enough that you couldn't keep your hand too near the wheel.

Not sure whether it's the discs or the pads overheating, or both? The pads were down to 2mm in the middle but 4mm at the edges...I know I'll need to replace them, but I want to be sure the discs are OK first.

How can I get the pads to 'float' a little better? I've never had this problem with any other brakes on other cars before, whether 4-pots, 2 pots, or even single pot sliding calipers. I'd change the pads, but if the existing ones are dragging this badly already, what'll happen when I add another 10mm of pad width in there as well?!

Help! Is it just 'running in' where the old pads are bedding to the replacement discs?

rustybin

Sounds like you need to give the calipers a little bit of a refurb. Chances are the pads are sticking either due to crud in the guides or at worst the pistons. You should be able to tell by moving the pistons back and forth. If you can do that reasonably easily then it is most likely the guides for the pads. A good clean up with a wire brush and some copper grease should do the job.

Incidentally, I would be very wary of running worn discs with a set of worn pads for any length of time, particularly with that little pad material which has now been fried.

Hakon

My 4 pot discs seem to wear more on the inside too, I know the pistons are not sticking so I am not too worried.
I would remove all pads shims pins, clean the lot with brake cleaner etc. I would then clean the 4 pots thoroughly, the caked on crap can be hard to remove. I would use an old toothbrush around the piston boots in case of tears with a wire brush.
Asses the boots and movement of the pistons for sticking etc. Any probs here and a 4 pot service kit will be needed.

If all ok refit with copper grease and new pads as 2mm is a bit worn. I currently have £30 omp pads and are fine up to a certain extent.

Hope it helps

Hakon

rustybin

Inside pads always do seem to wear first. If the pressure is the same and the piston size is the same, why would that be?

silent running

Great answers, thanks all.

I have got a set of those £30 OMP pads on the way and I'm not driving it again until I've changed them. It was only when I took the whole caliper off and looked at the underneath that I saw the 2mm thick centre of the pads. From the outside they look like a good 6 or 7mm still.

I'll go and have a crack at moving the pistons now. I've been trying to avoid the need to bleed the brakes, but looks like I could end up having to...