For a long while I was having a terrible time with stuck “pulldowns” aka “the trip south” with my Cory vacuum pot. My seal was good but the coffee would start with a trickle and then turn into frothy foam, and finally stop alltogether. The amount of vacuum was incredible as the glass rod would only turn with some difficulty – I’m luck I never had an imploded carafe! I tried the “new” cory rod but it didn’t perform any better. I then used a my Demel’s diamond cutoff wheel to make a bunch of notches in the new rod but even that didn’t help! The most consistent solution I found was to tilt the rod right before pulldown in order to create a larger gap. This would get the pulldown phase down to 5 minutes, still pretty bad. And of course it would still get stuck about half the time forcing me to filter the remainder of the slurry with my french press.
It turns out my problem was with how I was whirlpooling the coffee as I turned the heat off. I liked how this method would result in a nice cone of grounds but for me it also packs the gap between the rod and bowl with fine ground dust. This dust would form kind of a mortar resulting in stuck pulldowns. I’ve stopped whirlpooling and my pulldowns, with my original rod, take about 30-60 seconds now.
I’m guessing that the whirlpool forces the smaller particles down to the bottom before the larger whole grounds. I do notice now that after extraction the dust forms sort of a slime on top of the grounds wheras with whirlpooling the dust wasn’t noticable meaning it was more evenly distributed, or perhaps mostly on the bottom near the filter. I have a low-end burr grinder which produces a lot of dust. Those who sucessfully whirlpool likely use much more consistent grinders.