[David Hodgson]

Richard, you did not mention if you machined the disc's, from your description you did not. Although it is 'common' practice not to machine disc's at each pad change, they should not be overlooked.

Disc's and pad's will wear forming ripples in the disc. New 'flat' pads ride on the crest of the disc ripple, and if there are more 'crests' on the left disc, it will have more friction/braking on that side hence pulling left. The greater the amount or depth of the hills and valleys, the more surface area that disc has, again more friction/braking.

The pulling may subside with pad bed-in, but a disc machine should cure this. Just make sure you then don't put rippled pads on the new disc.