Yes, but does it still slosh when shaken? The question is, has all the air been removed before the pump is turned on, or is it getting in there while it's running?
The reason I asked (after assuming you had bled the system of air) is because my own cannister gets a bubble of air stuck above the impeller and it takes repeated power cycling to get it to flush the bubble through. Actually turning the cannister upside down and back again a few times while it's running pulls it out of the pocket where it gets stuck also.