"overfill" meaning when you fill them?
That could be because you need to pull the juice off the coil when filling, you are filling them too quickly or they could be wet when you fill them (it would thin out the juice a little). Just water will not stay in any of my CE3s....not saying it can't be done, just saying that plain water doesn't stay in the ones I use. You dont need all of the juice to fall to bottom, just pull it away a bit. Banging them around too much may do more harm than good. You could just stop filling when it hits the bottom of the barrel instead of trying to hit the line too.
I actually stopped cleaning them as often, even with juice changes and honestly, they are fine.
Funny story, I gave my wife a CE3 (she has been using all cartos). First, she said that it didn't work - I pulled the post down a bit and tried it, it worked and gave it back to her. Second, she came back to me and said it wouldn't work - I put it on the battery and it didn't pull so I unscrewed it slightly, told her it works and gave it back to her. Last time, she said it leaked all over the place - I put it on a battery, it worked and I gave it back to her without doing anything. She has only had it for two days.
It just takes a little bit of time, don't over-think it and if you are having too much trouble with one, toss it and use a new one - it is less than half a pack of cigarettes anyway.