Thanks for your reply Gene. I agree, the staves should be strong in a
vacuum, but the heads might get pulled in if the vacuum pressure is too
great. I started the rack w/ a very weak vacuum, and increased the
vacuum as the barrel was being filled. Thanks for the tip.

Vacuum racking could probably be improved if a negative pressure relief
valve was added. Is there such a thing as a negative pressure relief
valve??? If so, know where to get one? Thanks, -David