Rear brakes - Partial Pad Contact on Discs?

[voxmagna]

I think I know why it happens but haven't got a solution. The inner pads were running true to the disc leaving no rust band caused by the pad surface not making full contact over its friction surface.  It is the outer pad that shows the problem, with a wide rust band the same both sides. I haven't replaced the discs yet as they show very little scoring or wear but I think the issue is related to the caliper design.

 

I've added a marked up photo showing contact between the inside pad and the caliper single piston and the outside pad which is pulled by the fork through the caliper pins. The caliper pins show no sign of wear or play.

 

I think what happens is the inside pad being pushed by the circular piston is able to centralize itself to the disc as it wears due to the relatively small contact area between the caliper piston and the pad backing. The outer pad pulled by the fork has a much larger area of contact at the edges of the pad backing and is unlikely to centralize to a disc as it wears.

 

If I replace both pads and disc I'm certain there will be even surface contact for a while, but since the fork on the outer pad won't allow the pad to centralize, I think the problem will come back.

[muddyboots]

On mine (when I bought the car used), the inner surface and pad were as bad, if not worse, than the outer.

Since I replaced the discs and pads, they've worn perfectly evenly, no bands of corrosion at all.