I have fitted S3 brakes and calipers to my Audi A3 TDI the issue I’ve got now is when I break hard my Back brakes Judder. I have new discs and brake pads can anybody help on why it’s causing this

At a complete guess I suspect the ABS system is rapidly releasing the rear brakes because they are locking. Does the S3 have a different brake bias to the A3? Or perhaps the wheel speed sensors are different?

Hell Mr. Newbie,

Another pointer to supplement Cliff’s comments:-

When you fitted the (non standard?) rear brake discs, are you absolutely sure the hub mating service was completely free from corrosion, which might result in the discs not sitting properly on the hub? 

Vitally important consideration, as you probably appreciate. 

Perhaps you could let us know. 

Kind regards,

Gareth.