I have had the gunking up rear master cyclinder and replaced it with a newy. Now the new one is finally starting to show the same symptoms.
Ie. it won't full return to the bottom of the piston stroke.
so i took off the top banjo and found with a little force I could push it back down to the bottom of its stroke.
Then I dismantled it and cleaned it and stretched the spring ever so slightly, all seemed good for a few weeks but I suspect the spring has relaxed again and it was beginning to not return. When it gets to a certain height it will no longer scavenge new reservoir fluid.
So I decided to look at the whole cylinder and decided that the poor part is the spring. Its just plain not strong enough. Further thought on the matter had me decide to find a spring that I could fit inside the existing spring. I sourced one at the local hardware.
Its a CSC C612. ( I tried the C614 but it was a tad too strong so i backed it off to a C612 ). The great thing is that this spring can be fitted merely by removing the top banjo bolt, dropping the spring in then reattaching the banjo and bleeding the banjo.
Voila its great now. Didn't even need to clean it! or pull it off the bike.