antonj Okay looking at the picture you posted it would seem that you don't have Avid brakes as those are the only ones using that conical washer system. Loosen both bolts that hold the calliper down. Tighten only one side, should it move while tightening it is not the end of the world. Now shift the calliper either to the left or right to get it centred at the end where it is still loose. Once centred tighten down. Now it shouldn't move due to being held in place by the other side that is holding it down. Should you now notice that the end you first tightened down needs to be moved just leave the opposite side bolted down, undo the side that needs attention and shift as needed. With this back and forwards between the two bolts you should be able to get that sucker bolted down, slap bang in the middle. Hopefully you can make out what i tried to explain. Well this is what I do when I encounter a stubborn caliper that just won't centre and so far it has worked for me.
