Couple notes to go with my original post...
Since posting this I have taken out one of my newer carts that came from china a couple months ago. The filling in these carts is way too loose to do this technique with. The filler is not the same as the stuff that used to come in carts. This stuff has kind of a vertical layering to it. I could tell as soon as I pulled it out that it wasn't going to work. I tried it anyway and it didn't work. It's just so loose that the
juice runs through it.
If your carts have this type of filler, you are probably going to have leakage/flooding issues. The method requires the filler cap to be somewhat dense (like the filler that used to come in carts or the Walmart polyfill stuff). Normally (if the filler actually fills the cart) you would want loose material (like blue foam, aquaclear filter, etc). In this case you want dense. Because the
juice is free to slosh around under the filler cap, it will still keep the filler cap wet even though it is dense.
The other thing to watch is that the poly fits the shape of the cart opening snugly. If there are any holes or gaps around the edge, air will get in, juice will get out.
One other thing... when I 'top off' (it's not really a top-off because the cart is dry when I do it), I pop the cap out fill the cart with juice to just below the rim and then replace the cap. Because the cap material is densely packed, the juice will not run through it if you just drip on top (that's the same reason it doesn't run out when it's full).
Also someone asked what keeps it in contact with the bridge. The answer is nothing. The filler should be flush with the top of the cart when you put it on the atty. The bridge will then push it to where it needs to be, and it will just stay there.
I will try to get some pics later to show what it should look like.