I found that the polyfill that came in the pre-filled carts didn't last long at all. They seem to compress and get boggy. So, if you are still using the original cart-filler - toss it. Yeah - I wasted SO much juice in the beginning trying to get the cartridge thing right!
I didn't quite understand your explanation - you are putting e-liquid into an empty cartridge, then keeping it in a case? If so - I'm not surprised it's leaking
Give the Fluval a try, it's really easy and there are even a few videos on youtube that demonstrate how different people do it.
I found a way that works just fine for me. I use my Fluval-carts for about 2 days max. It's cheap enough so I don't feel bad tossing it, and re-stuffing carts is actually kind of fun for me. Once you get the hang of it - you'll be on a roll and won't even think twice about it.
A couple of things I always keep with me - Tweezers and Paperclips. A paperclip with one side pulled opened is what I use every time I top off. I re-fluff the stuffing at the top of the cartridge. I also insert it down into the cartridge in one of the corners... and drip the e-juice right there. It makes an air channel that allows you to get more juice through the cartridge.
The tweezers just come in handy for lots of things. I can pull the stuffing out a little, then kind of rearrange it when I top off if it starts to look boggy.
Also, when I re-stuff my carts, I use the tweezers to insert the Fluval into the cartridge. I always leave a little room at the bottom of the cart. I don't stuff the Fluval all the way to the bottom. When you do it the first time you'll know what I mean - when you insert it with the tweezers you'll feel a point of resistance - that's where I stop. I hold the filler with my fingers and pull the tweezers out - then give it a hair cut
So- yeah - toss the polyfil that came in the carts. I couldn't wash it out and reuse it either. I even tried just rolling it in a paper towel to clean it, but it just didn't re-stuff into the cart the same, and didn't hold juice as well.
Oh - what you can do before you toss the pre-filled poly stuff - use it as a gauge to know how big to cut your Fluval. It's easy to eyeball - but when you first start making your own, a visual guide can be handy.
You're off to a good start!
I just realized that most of what I just typed - really didn't make much sense
Vape on!
