thanks for the input. i was not sure about putting carb cleaner in it in case it would react with the plastic and/or the o-ring on the plug, but i may try it. i think i will try simple green first.

yes, the newer tank certainly has a lot of weird chambers and passages in it. i would fill it and then shake it and it would then appear to be half empty. i could also drain it, then turn it, and more water/cleaner would come out. seems like a pretty intricate design.
when turning/rotating the tank when shaking it, it is clear that there are numerous chambers and passages in it.