This thread inspired me to take a second look at my XT 8020s that have a wandering bite when cold.
I ran about three bleed funnels through each brake, using a syringe at the caliper for a vacuum. The old fluid was filthy! I also extended the pistons a little and lubed them before resetting, and rigged up a fork mount on my garage wall to hold the bike vertically for the back brake.
They feel a lot better, and I hope will work a lot better.