'81 w/ATE calipers. I just replaced the clutch master and rear brake
cylinders, and want to flush, then bleed the whole system.

Any clues on how the ATE calipers are properly flushed? Do I flush
using the lower "drain" valve, then bleed using the upper one? I'm
guessing the lower valve has something to do with the fact that these
are dual piston calipers, unlike the sliding caliper ones I'm used to.

I did try flushing using just the upper valve, but then when I cracked
the lower to see what it did, noticed that I got some old fluid from
there.