Jane, just read your post. I'll admit carto tank success can be a work in progress, been there and done that. I did see some issues that you shared.
You
don't want to take the top cap off the tank to fill. When you put it back on with a full tank, the pressure of the compressed air and juice takes the route of least resistance, which is to flood the carto and into the air hole resulting in leaking juice all over your mod.
A much better way is to first prep your punched cartomizer just like you would if you weren't going to put it into the tank. Make the polyfill nice and slushy looking. Putting a broken wooden tooth pic into the airhole helps keep any juice from accidentally getting into the air hole.
Then, you should use a tank accessory, or tank filling tool like this one, on the top of the carto:
View attachment 129268tank accessory tool from Big Daddy Vapes - the Slap Yo Moma
This device does 3 things:
1-It will help guide the carto thru the end caps of the tank during assembly. A little juice on the tool and the carto helps lubricate them to make passage smoother.
2-The tool also prevents the sharp edges of the carto from damaging the rubber o-rings in the end caps. These are what hold all the parts of the tank together, and if they don't work correctly, you end up with a leaky or loose tank.
3-You put the carto with the tool on top of it about 2/3rds of the way into the already put together tank, from the bottom. There will now be a space in the top of the top end cap to drip your juice into. The tool prevents any juice from getting into the air hole.
Leave a little room at the top, meaning don't fill to the brim. Then push the carto the rest of the way up, remove the tool, and replace it with your drip tip.
The tank filling tool is indespensible, and I believe a key to avoid user error when filling and assembling a carto tank.