I've given up messing about with this as its a battle I can't win.
No off the shelf EPDM hose will match the bend radius of the factory hose so I've covered it in adhesive-lined heatshrink. Polyolefin heatshrink is basically the same as polypropylene which is what the reservoir is made of so it should be resistant to brake fluid on the inside and waterproof on the outside. I think water vapour / condensation was allowing the fluid to get out by osmosis so hopefully isolating it from that will solve the problem.
I've tried cleaning the black streaks out of the reservoir with everything I have in the workshop: ultra-aggressive degreaser, acetone, celly thinners, contact cleaner, wheel cleaner (phosphoric acid) and even hydrochloric acid. Nothing has any effect whatsoever
They come off easily with a cotton bud, but of course you can't get to 99% of the inside of it so it looks like they'll be staying. I'm just going to put it back together and see how it behaves.
Not the most successful part of this restoration
