The PTB do still work. I too love the flat tips. I was not fond of the flat tips that are made for the 510. They are only partly flat. Not really flat like the 901 carts. Anyway, I have been using the 901 carts on all my 510's for a while now. So far the best filler I have found is the PTB. What you have to do is use the whole teabag. Fold it in half lengthwise. Then role the whole thing up tight enough to get it into the 901 cart. Trim off the excess. Now pull it back out of the cart not quite all the way. Use a very small pointed pair of scissors (like embroidery
thread scissors) to make several vertical (top to bottom) cuts in the filler. What this does is kinda makes the filler look like it's been
through a paper shredder. There is another post somewhere in the cartridge
mod section that explains this better than I can. But the end result is the same. It does make a huge difference in the whole wicking factor. So far after 3 weeks of doing this with mine I am totally satisfied with the vapor and the wicking.