The ABS pump is a two-cylinder fixed displacement unit which is designed to be primed with a normal system fill. Any trapped air bubbles rise clear and up the feedlines for later bleeding (this is why the pipes to block are down-feed from above and the main Reservoir is mounted at higher level).
Having said that, a full test of function could release some stubborn air bubbles and let them rise.

 

Good thought: The relative levels of the reservoir and the ABS unit make perfect sense for self bleeding. It wouldn't be helpful for any air trapped above the ABS in the two feed lines to be forced into the ABS block, which is why I bled that item first. Then I attached the caliper lines. Next, I connected both of the feed lines, and then put the pressure bleeder on the reservoir. When I cracked the rear brake feeder line I got a hiss of air followed by brake fluid. The front feeder line was not as noticeably air bound, for some reason. It may be, given enough time, the air trapped above the ABS block could be slowly replaced by fluid, and for the air to end up in the M/C reservoir, but it certainly didn't happen quickly for the rear feed (nearest to the M/C).