Uhhh...guys. The strength of vacuum is not in the reservoir. It is in the lack of air in a confined space and the connection between the inside space and the outside space of that confined space. The atmosphere provides the pressure. A reservoir simply adds size to the confined space - not always helpful. There are applications for reservoirs in very large and production environments. Our shops are not that environment..
I may have said that wrong, but the reservoir in the shop vac does play a part in creating the vacuum... you're correct that the hold is created when the air is sucked out of the inside of the turning, but with a shop vac, there needs to be a place for that air to go to... also you with a shop vac vacuum chuck, you need a continuous air flow, otherwise you dead head the vacuum and it will burn out.
I told Rob I would take some pictures of my home made vacuum chuck... keep in mind that it's pretty rude and crude, but it does work... and Jim Burr is 100% correct, it's safest to keep the tail stock up as long as possible...
Here are a few pictures showing my crude homemade vacuum chucks... one is a flat faced mdf disc with closed cell foam for pading, the other is a pvc fitting mounted on a mdf disc attached to a tapped piece of mdf that screws on to my spindle.
This is the front face... lines are for assist in centering the piece..
This is the back side of the mdf disc... I didn't round the mounting - I did say it's a little crude.... second picture is the disc mounted.
My vacuum tube is a length of clear plastic tubing, 3/8" id, the wood piece is turned to fit the intake port of my shop vac.
I should have made the lamp rod a few inches longer, it sometimes has a tendency to come off now, so I usually to add a clamp to hold the tube on the rod.
These pictures show my pvc chuck... used for flat pieces or an inside hold on a bowl with an uneven edge.... the black is a rubber gasket material to create a cushion... sometimes you can get a black rubber mark on your piece if it slips any... may have worked better without the gasket... the pvc fitting is a 4 inch coupling that is recessed into the mdf disc and glued in place with epoxy... silicon might work just as well, but didn't have any at the time when I made this.