I've not cleaned out my Venco Pugmill for several years and it just seemed like the time to do it because I was not getting a good vacuum and there looked to be lots of corrosion. I had decided to install Buna N rubber gaskets all the way around to try to get the vacuum tighter, and to try to have less air seeping through the joints. When I don't use the pugmill for several months there is a noticeable dryness at the horizontal seam that runs the length of the pugmill. Venco recommends electrical tape for the joints, but over time, air gets through and it causes problems with stuff spalling off the sides into the clay, and the hardness I mentioned.
One test that I used was to take a pug and twist it, and in so doing there were air bubbles and delamination. So I decided to tape the screen aggressively so that the total amount of volume that needed to be deaired was less than before, and was nearer the vacuum chamber. As luck would have it, Rick McKinney of MKM Tools stopped by and he noticed that the screens were polished where the most clay went through and suggested that I tape with this in mind and so I did. Not sure how this will work, and for sure the mill will be slower, but I hope it helps. The mill is too fast anyway -- hard to keep up with for one person.