I have tried all the cart mods (PTB, ZFM, bluefoam, whitefoam, spring, straw, etc, etc). What I am using right now on my 510 is working like a charm.
I took a new cart, pulled the filling out, and cut it so it is about a 1/2" long. Then I can put about 10-12 drops in the empty cart and put the little piece of filling on top. Then put 2-3 more drops on it to get it saturated. The little piece of filling stays soaked until the entire cart is dry.
The other thing this does is allow the bridge to push the filling down into the cart a bit so there is very little contact with it. This seems to help to not get the 'burnt polly' taste that most filler mods give.
* I should mention, I am using this with my Chuck so it sits batt-side-down 90% of the time. Don't know how well it will work on a stock PV that lays on its side most of the time.
It works great on a 510. I did this by accident yesterday my filler ended up in half so I only used half. I was shocked that that I wasn't refilling or topping off like I was with the full filler in.