i would think if the pump doesn't work, and the fluid isn't moving, then the air isn't going to come up and out of the rez.
are your sure your rack and/or the cooler isn't the problem, and some how causing your pump to fail. it seems to me that after having 3 pumps stop working, you either have a horrible parts supplier, or something is cause the pump to fail. in this case it seems like the rack and/or the cooler are the causes since they are the only things connected to pump.