Brake judder can be caused by a number of things, sometimes difficult to accurately diagnose. Most often it will be because the disc does not run true in the calliper, this could be because the disc has warped due to excessive use (quite rare) or was fitted incorrectly (more common). Another cause on older car is where the disc has corroded unevenly, particularly if the car has stood for a while, sometime the area under the pads remains untouched whilst the rest of the disc rusts.
Fitting discs is not just a case of bolting them on and hoping for the best, any slight unevenness where the hub meets the disc can tilt the disc slightly, a small error near the centre makes a big difference near the edge of the disc, anything over 0.1mm of variation is a problem. If there is any slight tilt in the disc it will gradually worsen as it wears unevenly, initially it might feel fine but typically it becomes a big problem between 2000 and 4000 miles after the discs have been changed.
For this reason after you have removed the old disk it is vital to clean up the mating surface, initially clean off all the dirt from the area then using a flat edge ensure the mating surface is completely flat. It should only take a moment to do but makes a big difference.
It’s remarkable how many ‘professional’ places don’t do this.
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