I've just come to swap the rear discs and pads on the back of my S4 (B6) - what a faff!
I searched here elsewhere and I couldn't find one thread that covered swapping rear discs and pads on these cars - when I had a B5 S4 - couldn't move for threads :-) I guess nobody likes these V8 ones!!
In case anyone else comes along looking for it - here's what I did:
13mm spanner (ring or open ended - I used ratcheting ring)
15mm spanner (open ended)
8 MM allen wrench (Big looooong one - you're going to need the leverage!! - I used Halfords Professional set - 19.99)
Thread locking compound (I used Loctite in a red tube, blue fluid)
Brake / Clutch cleaner
First, loosen wheel nuts (slightly - 1/8th turn will do)
Jack car up (I reversed mine onto wheel ramps so I could get a trolley jack underneath)
Remove wheel (duh!)
Using 15mm spanner, hold the caliper carrier slide nuts, then undo nut holding caliper to slide with 13mm spanner.
Swing caliper out of the way
Remove pads from carrier (inside pad is prob way worse than outside e.g down to the backing whilst outer has 4mm meat on it)
You should now be able to wiggle the old discs out of the way, it might take a little jiggling, but nothing too hard.
Now for the pain in the ass bit, Audi decided to use allen headed bolts to hold caliper carrier to hub - and put in such a way you can't quite get a ratchet and 8MM allen socket in (damper is in the way) so you're going to need a looong 8mm hex wrench. I say looong as it's torqued upto 75NM and plus the corrosion ;-)
If you managed to wiggle the old discs off, I recommend spraying penetrating oil onto the threaded end of the 8mm allen bolt (you'll see it) and give it a few whacks with a drift (or a 1/2" extension bar) as it's likely siezed in.
Put the short end of the allen wrench in the top calliper carrier bolt, make sure it's in there good - don't want to wreck the head - and apply enough torque to get it loose - I wouldn't bounce up and down on it - but build the torque up progressively - I got both mine undone without resorting to breaker bars etc.
With a bit of luck it should have come loose and you're in business.
Try as I might I couldn't get the bottom one undone, the bolts were at such an angle I couldn't get any of my 8mm hex tools to fit in - but, with the top one undone - a block of wood held against the carrier and liberal application of a big f**k off hammer and it moved an inch - which was plenty to get the carrier out the way and the new discs on (new discs are approx 2 mm fatter than old worn ones - amazing how much thinner they get when they wear!). Before you put the new discs on, be sure to give them a clean with the brake cleaner - they're shipped with a protective coating which you need to clean off.
Don't forget to tighten that top bolt back up on the carrier! I coudln't get a torque wrench on it so had to guesstimate 75nm - I went with moderate force (ie NOT heaving on it like a nutter) till it wouldn't turn, then plus a 1/4 turn. Apply some thread locking compound before you put the bolt back in - a light line from top of thread to bottom - not too much ie dripping off!!
Now you need to wind the pistons back in - you can't really just force them back in like the front pistons - they need to be wound in - I used a 30 quid tool from somewhere or other - but I have seen people using a small G clamp. Whatever you use, remember that as you push them back in the fluid in the reservoir will rise, take the cap off to allow it to rise (or else there's a slim chance you'll blow some seals internally) and remember that brake fluid is corrosive to paint and your skin....
One the pistons are back in, you'll be able to pop the new pads in the carriers (I put a smear of copper grease on the backs of the pads - force of habit - anything to stop sqealing brakes) and the pistons will be far back enough to clear the pads as you put the caliper back onto the carrier. Again - remember the thread locking compound.
Lastly - put the cap back on the reservoir!!
Before you drive off, with the engine running - pump the brakes, get the pistons back in contact with the pads - you don't want to get to the junction at the end of your street and find you have no brakes!
Once you feel the pressure has built up again in the brake pedal - move forward a couple of paces and make sure the brakes stop nicely. IF it's all ok so far, go for a test drive - dab the brakes a few times BEFORE you really need them - make sure you're happy with them. IF all is well, avoid hard prolonged useage of the brakes for the first few hundred miles, drive normally applying the brakes normally - as we want the discs and pads to bed in properly. After 100 miles or so you should check your wheel nuts are still tight, (I torque mine to 110nm) - you should see the discs have gotten shiny as the pads are bedding in and your brakes should feel alot better!
Top tip - if you've bedded the discs and pads in ok - feel free to go back to having a blast! But, after hard braking, avoid sitting stationary with the now very hot brakes on - you'll be putting a lot of localised heat into the discs which can lead to warping etc. I tend to creep the car forward a foot or so every 20 / 30 seconds and reapply the brakes - in an attempt to even the heat out.
What surprised me is that ElsaWin doesn't cover discs removal at all - I can see caliper servicing - which shows the discs - but nowt about getting them off.
I can't believe how awkward Audi made getting the carrier off, but getting the pads out is an absolute doddle - if only the carrier was easier to get at.