As I said before, the braking system was not fully disassembled, the calipers were removed from the wheels and the pistons were cleaned without being removed from them, only pushed outwards with the help of the brake lever or pedal (all three at all calipers), up to the limit of full output, without falling out. I didn't remove any hoses. After cleaning, they were pushed, at the same time, to their place inside. Along with this cleaning operation, I also replaced the brake fluid from the entire installation (again), by pumping and filling successively in the tanks, according to the method described in the brake bleeding section (in the manual), using about 1.5 liters of fluid, until the fluid came out as clean to the aerator, like the one put in the tanks.
The temperature grease, which I used to grease the sliding parts of the calipers, marked in red in the attached image, is Mannol 8025 brake grease (I didn't know exactly than, because I received it a small amount in a box, from a friend).
The brake disc on the rear wheel is scoring and discolorated (blue), indeed, and measures only between 5.5mm and 5.9mm thick, so far below the limit of wear.
Just as I said before, when the brake lever or pedal is actuated, the pistons actuate the pads on the disc, and when the levers are released, they retract slightly into the caliper, releasing the pressure applied to the pads.
Looking for rear brake discs, for the 1998 ST1100AW model, I found them in three places, with quite significant differences in price and delivery time. The most expensive disk is from MTX, stainless steel, thickness 7.8mm (124.99 GBP + 32.66GBP shipping + 31.71GBP custom duties, delivery 12-14 days) and I found it here:
https://www.ebay.com/itm/265222836414?hash=item3dc08350be:g:2lQAAOSwX95g5r8j&amdata=enc:AQAHAAAA8EUVcbQaNjAHSKYzB2zy++7uhoMZsruaASZNXC9Ctt+lWgZFZ/dPBW42r66L7I4p+NwxSauQOvL99t+Y1XOiOEkn61zPMzqWIuzJLcQuyUBQThC6/ELrxtMKUZXNv5y1ZdZs3nVyCEf6Jl3AvJN3KzhHQRPqTHq+0KcUW1ylKDV2ECmFqWUPRVTmnPBiBxw3HTPkar8Ud2+SpSHeQBsPTNDke4wzQHrxpZcOQ7ZK2msU8OHz0ysaiowarlGQzo2gkMf40oGAt503pjumFJewTnTYXaiZ58HbsTXD0mT3Sqfq8vjJN1fRfI/FcI6Xpr6jdQ==|tkp:BFBMuqDP9thh
The second found on aliexpress, BIKINGBOY, stainless steel, thickness 7.8mm, a bit cheaper (102.24GBP + 21.79GBP, long time delivery between 30-45 days):
https://www.aliexpress.com/item/1005003954012844.html?spm=a2g0o.productlist.main.21.d9a952b09wNJAC&algo_pvid=a62e1cdc-2e84-48f8-a7f3-ddfb0260cb22&algo_exp_id=a62e1cdc-2e84-48f8-a7f3-ddfb0260cb22-10&pdp_ext_f={"sku_id":"12000027556294692"}&pdp_npi=3@dis!RON!736.47!559.74!!!!!@21227d8316783832335797406d0705!12000027556294692!sea!RO!0&curPageLogUid=p8H0zXRNjBDs
The third and cheapest is from Niche Industries (59.95 USD + approx. 40 USD shipping, delivery 15-20 days). I haven't heard of them, I don't know what quality they produce, the disk seems to be made of cast iron and thickness 7mm, but if you have used components from them, any advice is welcome. It is this one:
https://nicheindustries.com/products/niche-519-crt2576r?_pos=1&_sid=3c67ac959&_ss=r
The disc from EBC is even more expensive than these three listed, but in the last resort, it can be considered. They currently have none in stock.
What would be your recommendations?
Thank you.