You can usually tell a warped disc through the pedal, It will be related to speed and basically pump in and out as the pistons get pushed in and out as the disc oscillates. I don't mean a disc that is thicker in places, but warped like the brim of a hat. From memory the 90 doesn't have 4 pot calipers on the front. This is a common cause of disc warping as opposing pistons that have failed allow the others to 'bend' the disc. Discs can warp for other reasons of course. But you really would be able to feel that and not sure it would cause severe swerving. Older discs tend to get a ring of old crusty stuff around them making Shayne's test difficult to evaluate fully. If that is the case then a dial gauge and magnetic base would be another way with the disc still on the hub. The only other thing I can think of is mismatched discs. No batch of metal is the same - it's within tight tolerances, but never the same. If you have one genuine and one Milner, say, the actual braking charecteristcs can be widely different. I have had matched pairs of discs that are poor in terms of stopping power and some that are great. If it had a replacement disc on at some point, it could be that it's better or worse than its companion leading to the pulling effect.
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