Before you begin work know the dangers of asbestos dust. If one of the discs requires renewal, both should be renewed at the same time to ensure that the braking is even and consistent, and new brake pads should also be fitted. Starting with the front brake pads, remove and check for cracks or deep scoring on the disc friction surfaces, light grooving is normal and can be ignored. A cracked disc must be replaced while a scored disc can be reclaimed by machining if the thickness is still above the specified minimum. Check the disc run out by a dial test indicator placed near the outer edge, if the run out values are higher than the figures stated then machining may be possible, otherwise a renewal is required. Excessive variation in disc thickness can be a source of judder, so take the thickness of the disc using a micrometer. After removing the brake pads and caliper, undo the two mounting bolts to remove the brake caliper bracket and limiter bracket with the note that new bolts will be required for refitting. Check whether the location of the disc in relation to the hub is marked; if not, make your own mark for refitting. Remove the bolt that holds the disc to the hub and remove the disc. With a new disc, ensure that the hub and disc mating faces are clean and remove any rustproofing compound with degreaser and a rag. Align the disc on the hub with the orientation marks and re-fit the retaining bolt and tighten it. Reattach the brake caliper bracket and limiter bracket, tightening the new bolts to specified torque, and then finally refit the brake pads.