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Working on the Rear Brakes - Retracting the Caliper Piston

When you change the rear brake pads you need to retract the brake piston into the caliper.  This is because the new brake pads are thicker than the old, and so the piston has to be pushed back to make room for the pads to fit around the brake rotor.  For the front brakes this is not too difficult - you can simply push against the piston and it retracts - be sure to open the cap on the brake fluid reservoir first!   But the S5 has an electric parking brake which uses a motor to operate the rear caliper pistons when the parking brake is engaged.   I believe that this motor also facilitates the brake pad wear indicator - the system can tell how thck the pads are based on their position when the parking brake is on.  Consequently you can't simply press against the piston to get it to retract.
There are two techniques you can use: either manually press and simultaneously twist the piston, or better yet - use a tool such as the Ross-Tech Vag-Com or the Mac Tools EPB101 tool to send a command to the parking brake motors to cause them to fully retract.  I've done it both ways, and the latter method is highly recommended.

Method 1. Using Vag-Com or the Mac Tools EPB101

These are tools that  plug into the car's OBD diagnostic port (located under the dash, below and to the left of the steering wheel), and that can then command the car's electronics to retract the rear caliper pistons.  The Vag-Com Diagnostic System is a powerful tool that uses a special cable to connect your laptop PC to the OBD II port.  You can then use the Vag-Com menu to control all sorts of functions - such as altering the way that the DRLs "wink" when the turn signal is on, or disabling the seat belt chime.  The cost of the fully-supported version from Ross-Tech including the connecting cable (coded HEX-USB+CAN) is $349.
An alternative which is significantly less exensive but limited in functionality is the MacTools EPB101 tool.  This device plugs into the OBD connector, and its only function is to retract the rear caliper piston.  Once you're done servicing the brakes to you then reset them for your new, thicker pads.  If you don't need all the other functions that Vag-Com offers this could be a cost-effective alternative.

Method 2.  Manual Push and Twist Technique

The manual technique which works on many German cars involves pressing the piston while simultaneously turning it (clockwise).  There are tools that are advertised as being able to do this - from companies like Harbor Freight.  These tools put pressure on the face of the piston, and with an appropriate adapter that mates with indentations in the face of the piston you can twist the piston while pressing it.  The tool comes with multiple adapters that are supposed to allow it to work with all models.  However, I found that Audi has out-smarted these guys  - the pistons on the S5 have a face with a unique set of 3 indentations spaced at 120 degrees around the edge,and none of the adapters that come with this tool will fit that particular configuration.  The technique I used was to use the flat-faced adapter, so that the tool simply presses without turning, and then I used a pair of channel locks to grab hold of the exterior of the piston and turn.   This is very time consuming and a major PITA!  You need to be very careful to not damage the rubber boot around the piston.