Hey Robert:
I have the Dura-C, I haven't looked back , but I think you have a 510. The high temps seems to cook and thicken the liquid. It thickens from the high heat of use and stops the cart from absorbing the liquid. Thus, we need to replace the cart filler when this happens. The best I have found, (closest to the origional) filling. Would be quilt batting. Go to a fabric/sewing center or store. It is sold on a bolt (a big roll). You can buy a 1/4 yard or 1/2 yard. Look for it to be thinner and all the fibers contained in a sheet. Do not buy a large bag of pillow stuffing (Big and fluffy, as many do here). It has too many loose fibers.( and god knows what your date thinks what you are doing with it in your closet when she comes over. ) But seriously, it is definitely not the right stuff. (I sew, quilt, craft, and that is not what is in the cartridges.) Take a look at it yourself, it is sheet looking, Not a big fluffy wad.
Don't get flustrated I can help!