In reply to Dave Todd:
> (1) Get the 'pro-level' bleed kit - the basic bleed kit is awful.
I've got the pro-level bleed kit and I still thought it was rubbish - leaked air between the syringes and tubes. My top tip would be to glue them together before you start, so they can't leak.
Mines a few years old, though, and looks like they might have updated it.
> (2) Spend ages making sure that you've got all the air out of the lever end of the system. If need be tip the bike up on its back wheel (like a 90 degree wheelie) and pump the brake lever for a minute to see if there's any increase in spongyness - it there is then extract the bubbles from the lever again. Repeat as required.
I've not done Avid brakes, but have done the Hayes ones on my sister's bike.
If all else fails, usually injecting a full measure of fluid from the caliper up to the level will help.
Also the "pull fast, release slow" technique of pumping the brake lever can help clear air bubbles.
Apart from the leaky pistons mentioned, I once had an issue with a pin hole leak in the front brake hose on my bike - replaced hose, bled it, problem solved.
When I get time, I'll be upgrading the mechanical brakes on the CX bike to hydraulic - that'll be a whole new world of fun, I expect!