is the reservoir attached directly to the pump or stand alone ? if its not attached then the feed line going out is usually at the very bottom and likely you cant even suck the fluid down that far. even if you did, the line is still full.
did you happen to start the engine while nothing was in the resorvoir ? that certainly will put alot of air in the system. otherwise just sucking the resorvoir doesnt introduce air.
its easy to get air out. wheels off ground just turn lock to lock few times with eng off. lock to lock and hold for a second or so with eng on.
of course your aware the draining resorvoir is only good for dilluting the remaining system