Recently dealt with a bunch of stuff on my passenger caliper and figured I would detail the info here in case it might help someone in the future.
About a month ago my caliper sprung a leak. over 200k miles on the car so it didn't surprise me. Purchased the Audi rebuild kit for it that came with new seals, dust boots and new pistons. Replaced them all and bled it and it was fine.
Fast forward to last week, I was sitting at a light and the pedal started to drop. Fortunately I was close to home so I got it into my driveway and parked it. Initial thought was that I messed something up on the rebuild, or that the brake cleaner I had used ate away a seal or something. Tried to order a new seal kit but they were back ordered everywhere. This lead me to searching for just the seals sourced by Brembo or Stoptech, which I found.
The D2 Audi S8 caliper is technically the Brembo F50. It has two 44mm pistons and two 40mm pistons. The replacement seal kits are
143.99040 for two 40mm piston seals/boots
143.99044 for two 44mm piston seals/boots
I found those and got them shipped to my place rather quickly. Tore the caliper out and put all the new seals in again making sure everything was perfect. Bled the caliper and dropped it down, went to move the car and the pedal was still dropping...
I could see it was leaking from the caliper somewhere so I enlisted the wife to press on the brake while I eyeballed the caliper, found out the culprit was the hardline "crossover pipe" AKA "bridge pipe"
Fortunately with this version of these calipers, there isn't a passage between the caliper halves needing an o-ring. So there is no reason to ever need to split these calipers.
The hardline bridge pipe can be made fairly easily. The fittings are the standard metric M10x1. Originally they used a 10mm hex/flange wrench. The only thing available locally were fittings that use a 12mm wrench. There's enough room for them but you are stuck using a regular open end wrench. The hardline is the standard 3/16 pipe. These also use bubble flares.
For anyone doing this, to make a bubble flare you need a "double" flare tool and just skip the last step. If you don't have experience with using a flare tool, do a few practice runs first.
For the sake of information, in my searching I came across a few pictures that helped me out. I've saved them and rehosted them.
Here are the insides of the calipers. these might not be our exact model, but it's the same setup.
Here is the crossover tube. my flare wrench wouldn't fit in there so I used an open end wrench. One broke free pretty easily so I slid the box end over the free fitting and around the tube to the other fitting. My tube was leaking from behind the rubber spacer/sleeve. I did not put that rubber piece on the new pipe, my reasoning below.
New pipes are available, but there are a number of sizes for the different Brembo setups. These pipes pictured are from a retailer that lists these for Mitsubishi Evolutions.
When bending these up (using a brake line bender of course) I started with the full 90 degree radius on the right side, then working from right to left I did the second bend approximately 60 degrees ensuring the fitting was on the correct side of the bend. The left most bend of about 30 degrees was done after that fitting was finger tight, the whole tube was bent over so the opposite side lined up. I did this because the last bend was so close to the fitting that the bending tool wouldn't fit, and you couldn't move the fitting from one side of the bend to the other after it's bent.
I skipped installing the rubber piece because I believe it was used to help bend that pipe upon installation and locate it for mass production. Since the majority of my new pipe was bent on the bench, and there is space between the pipe and caliper once installed, I didn't see a need for it. It also provides a home for water to accumulate and corrode the pipe, so I left it out.
Hope this helps someone!