It's worth putting the battery on a trickle charge doing rear brakes, with or without the scan tool.
As mentioned, VCDS can cycle through the service of these brakes with the electric handbrake. With the electric motor disengaged - the piston can be retracted with a 7mm allen key, then pushed back with a G clamp. There shouldn't be much resistance. If you do remove the electric handbrake - make sure the T30 Torx heads are clean of rust and corrosion. Spray on a little PlusGas. If you round the head on it - you're in a world of pain.
I rebuilt the rear brakes on my old B8 Allroad. Audi don't even sell rebuild kits, they may for C7, but I suspect just the fronts. The kits are available on Autodoc (Budweg) and include the rubber 'o' rings for the caliper and electronic brake, as well as rubber dust sleeve, square seal, etc. Those 'o' rings are critical - as the calipers do suffer from corrosion, where you get binding brakes, etc. Check the old discs for heat spots, and the pads are equally worn. Pretty sure from memory you will need long nose needle circlip pliers to do this service if you choose to rebuild.
Personally I'd remove the brake carrier. Half the time you get issues with rear brakes is because the shims/springs are too tight on the carrier due to corrosion. You can use a metal file to clean them properly and a wire wheel attachment on a drill. Use a little ceramic brake grease with the new pads shoulders and on the piston face. I also use it on the outward face brake pad back where the caliper touches it. I used to use red grease on the brake slider bolts - now I use ceramic brake grease. Make sure pins are spotless. You should get 13mm bolts in the pad kit with thread lock already applied.
I know this is for the B8 - it's one of the best videos out there in terms of the process.
View: https://www.youtube.com/watch?v=urUIx-kq1O4&t=1141s
Look at the state of this, not even wire wheeled the brake carrier, running old discs with new pads (I know the rotor was on order), no ceramic grease, brake springs or shims added to a crusty carrier and the pad is tight as a gnat's chuff. That's brake issues in no time.
View: https://www.youtube.com/watch?v=lf-JNLZ2ncE
