I eliminated this type of problem years ago by using a big bore blowoff tube. This is a chunk of the 1" i.d. vinyl hose instead of the usual 1/4-3/8" tubing. You can't create enough suction to draw any liquid back into the fermenter. The big tube fits tight into the neck of a carboy, eliminating the need for a stopper.
Actually, I haven't had the need for a blowoff tube for a couple years now that I started using a little Fermcap S in the fermenter.
