I was brake-bleeding last night using a Little Bleeder (for those of you outside the UK that means a plastic tube containing a one-way valve to avoid drawing air back through the bleed nipple during bleeding), and I've had some problems.
First I flushed & replaced the old brake fluid, taking care not to let any air in. Brakes tested, all good, smiling like a Cheshire cat.
Then I had second thoughts & decided to clean the pistons & caliper, so I had to go through the bleed process again, this time from an empty system. This is where the problems began: after pouring in new fluid into an empty system I can't get any pressure in the system when squeezing the brake lever - it's all spongy. I know the calipers are full of fluid because my Little Bleeder draws up some into the plastic tube immediately.... but it falls back into the nipple straight away.
There's obviously air being drawn into the system, and I think it's through the threads around the slightly-open nipple. I've tried everything to get rid of the excess air, from the old-fashioned way (open nipple with half-turn, squeezing brake lever, close nipple, release lever, repeat), through to quick pumping until there should be no more air in the system at all (or my lungs).
I'm going to try to find a way to wrap some teflon tape (if I can find any) or blue tack (as long as it doesn't stick to the threads) to seal the area around the nipple.
Does anyone have any other bright ideas?