HI entropic, Try using an unbent paperclip in the whole at the mouth piece end and push the cartridge up.. You can then fill it via syringe - just fill it till its wet looking - but dont let it pool on top = juice in the mouth)..
Some people will then use the paper clip on the side of the catridge to push it back down...I found that when I did that , half the time I was smashing the filling back down..So my hubby found a little wooden dowel rod- that was the exact same diameter as the 401 cart..He cut it in 2 inch pieces..I sanded both ends flat..Now when I fill it I can use the dowel rod to just shove it back in..It's lightening fast.. I was thinking the lil plastic caps that come in the carts might do the same..I havent tried that..
If youre replacing your cart filler material.. You might also want to play with the amount you put in your cart..Too much = draw will be too tight, wont wic properly ect..
There are two videos out here showing both these techniques ( look in the newbie videos, newbie forum)... I don't know how to do links or I'd hook you up... Good luck