I got the PSD unit off that GTS. I fitted it today and then bled the system and slave cylinder.
I used the info from Louis and Jon's excellent write-up. Thanks to them for sharing!
I added more info/pic's that will help other people do this service work.
PSD service work is VERY important. Without clean/fresh brake fluid, the pump seals wear and leak. The pump also develops bore wear and is ruined. This happened to my S4 as it had not had the PSD serviced regularly and by the time I bought the car back in 2014 the PSD was already leaking. By 2015 the pump had leaked all brake fluid out and then failed.
I put a decent 2nd-hand unit in and then did the full bleed. Here's some helpful pic's/info not on Louis+Jon's write-up.
1) Getting the PSD unit off is relatively straight forward. There is 2X 10mm bolts into the inner wing; a top one and a bottom one. Then there is a 3rd 10mm nut that is hidden behind the pump. You can only access it with your hand pushed under the pump and fiddling about with your fingers. Once found, use a long 10mm socket and small ratchet to remove it. When removed you will see there is a holding bracket that it holds firm at the back of the PSD. To refit the PSD unit lift it into position making sure the holding bracket is back in the correct place. You have to lift the PSD unit up higher than it actually bolts in to the rear inner wing to get the bolt for the back holding bracket to slide into the hole it is bolted into. Once done, loosely screw in the bottom 10mm bolt to hold the PSD unit secure and then screw on by hand the tricky nut hidden behind the pump. To do so, put your hand under the pump and feel for the thread and then screw the 10mm nut on. Then do the bottom/top inner wing bolts up tight and then across to the brake line that runs near the gearbox.
Wiring for removal: there is the wiring connector to the lock solenoid and then the plugs to the pump and ABS switch. There is also a brake hose that runs across to the near the gearbox (see pic's). Best to disconnect this and then remove it complete with the PSD unit as you cannot get access to remove the line at the PSD unit itself.
2) You need a good length of hose for the bleeding and somewhere to safely seat the collection container. I used a plastic juice bottle as shown in pic's. When flushing the PSD slave cylinder, again you need a long hose running off the bleed nipple and then tie-wrap the collection container as shown in the pic I took. This means it won't fall to the floor when left unattended when bleeding whilst you activate the electric side of the lock line.
3) For the wiring required to bleed the PSD lock cylinder and line, I made my own by cutting off my old connector to my old PSD unit, and then cutting it so it fits inside the wiring connector on the replacement unit. Using a 10mm nut, I then ran a ground connection to where the wiring for the brake sensor is attached to the hub and then used a spade connector on a super long wire to the positive on the battery and simply tightened it onto the battery terminal. I then pushed the electrical connector I made out of my old PSD unit into the lock solenoid on the replacement unit in the car and then reattached my battery ground, at which point the lock solenoid clicked open and bled old fluid out as it should for 7 seconds. I then pulled the ground off my battery and the job was done! By making this wiring I can keep it and re-use it for this service every 2 years. If you don't have an old unit, the electrical crocodile clips will work fine.
It's only taken me 5 years to get this work done!
Hope this helps other 928 owners.