The instructions I read were unclear to how to attach the through tube to the faceplate. Since I had epoxied the tube into the outside part holding the bearing, I decided to use a rubber washer and nut. I found a conical rubber ballcock connection washer (used on the water supply to a toilet) and it fit the lamp tube and worked well.
Then I got an attachment that piped the vacuum into the headstock shaft with no through tube. I found that the same conical washer fit into the #2 MT protruding about 1/8 inch. So when the faceplate is screwed on it presses snugly against the rubber washer. With this setup I am getting the best through vacuum of any setup I've tried. I did have to do a little adjustment. My particular faceplate barely touched the washer before bottoming against the shoulder of the shaft. This was fixed by epoxying a metal washer onto the bottom of the faceplate inside the nut.
Hope this helps some of you who are struggling to do a home brew setup for vacuum chucking.