I know some folks bleed some air through into the vacuum hose to allow the motor to breath a little. However, note that a vacuum cleaner doesn't pull nearly the amount of vacuum you need to do a true "vacuum bagging". It will help you pull the fiberglass into complex corners but it will not pull the airbubbles and voids out of the layup. To start to get the advantages of vacuum bagging, a vacuum level of at least 10in-hg (I usually pull 18-20) is needed. You get maybe 1 or 2 (I'm guessing) with a vacuum cleaner.