It's been a few days since you posted your helpful comments.... among those was to be sure and use quality refillable carts in good shape because the rubber (plastic? Neoprene?) seal at the exit port can age and allow air to enter... thus causing air blockages which give the appearance of a blog. Here's another possible cause for a poor seal at the exit port:
I ran across this post on another forum and the writer suggests cutting away excess cellophane around the perimeter of the exit port, thus allowing a more air-tight seal. Go down the posts and see the photos posted by one Larryb:
https://www.printerknowledge.com/threads/an-easy-clog-to-clear-on-one-cleaning-but-still-a-nuisance.11243/page-4
What do you think? I experimented with a defunct cart. to get the hang of it. Haven't tried it on my cart in the K position. Looks promising.
Paul W.