"Moray" wrote: (clip) the holdfast system which produces 19hg. A used pump
I have seen
produces 22 hg. Is the difference a big one or is the 19 inches
enough.
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The difference between 19 and 22 inches will hardly be noticed.
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(clip) up to what size/weight bowls can it handle.
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Turn the question around--*DOWN* to what size will it handle. The risk of
knocking a piece off the chuck is greater for small bowls. I can't think of
any way to predict exactly when you will lose your grip, but for a given
article, 19" will hold 86% as well. Or, putting it another way, the
Holdfast system will have the same strength as the 21" setup at 93% of the
diameter. (because the area goes as the square of the diameter.)

What you need to compare for the two pumps is their relative pumping volume.
A pump that handles larger volume will deal with leakage better--you won't
have to worry as much about the seal, or the possible porosity in the wood.
Again, it is difficult to put numbers on it, but it is good to have a
capacity of 3 cfm or so to work worry free.