Most ABS pumps are open valves normally. The solenoids will cycle the valves to cut pressure. So really there is no air that can get in.
^^^^ This is true. Say, for example, you had to replace your master cylinder. A "regular old" brake bleeding will, in virtually all cases, purge all the air out of the system. Only when the ABS unit itself is disassembled do you need to cycle the system.