For rebuild parts for Brembo brake calipers, try: Bevel Heaven
Or: Munich Motorcycles
The pistons in the calipers are on "dead ends" in the hydraulic passages, unlike most auto calipers. The bleed fitting is connected directly to the brake line beside it. If crud ends up in the passages or pistons, you cannot flush it out by bleeding. Sorry.
I had a similar problem to the OP last year on my '99 R1100S. I disassembled and thoroughly cleaned both front calipers. Lots of crud from decomposing OEM brake lines acted like a check valve inside the calipers. Pictures here:
There are warnings in several places to not disassemble (split) the calipers obviously written by the BMW attorneys, not the mechanics. Well, these are MY calipers. They are not much more involved than the oil drain plug, actually. There are four regular bolts and one small "O" ring in a cut-out between the halves. If you can't handle "O" rings, then I too suggest you not disassemble them, but that is the ONLY way to clean and verify they are clean inside. Just only do one caliper at a time. All of my parts, rings, seals, pistons, were pristine (see pics), didn't need anything but DOT4. No problems or leaks since. Rode 800 miles to the Twin Valley Ralley in Meadows of Dan, VA this weekend plus another 4K since the disassembly.