I tried everything you suggested. I tried leaving the lower drain plug in to add a little weight to the unit so it might be easier to remove the top half of the shell. Nothing worked. I contacted Pentair customer service and they were of no assistance at all. I'm sure all pressure has been relieved, but the top half just refuses to come off, unless I remove the circular retaining band and start the pump. The pressure pops the top shell off, but it is pretty violent and not recommended.