Try blue foam in your carts. For 510 there are precut pieces cheap at strictlyejuice.com and you can also cut plugs for smaller carts from them (I cut M401 and Joye 306 cart stuffing from them). Or buy the uncut at petsmart - Marineland Rite-Size U Foam Sleeve in the aquarium dept.
I think the blue foam wicks more consistently. I can definitely empty my carts but I really only use carts on M401 and on Joye 306 atty on my 510 which has smaller carts, and use cartos most of the time on KR8 and 510. The blue foam holds its shape, easy in, easy out, easy to wash in running water and pat dry. For a full fill you take it out, fill cart 1/2 way with juice, put the stuffing back and push it into the juice, reposition at the top end of the juice cup and top off with 1-3 drops - whatever it needs on top.