The vacuum systems which we use have been updated and upgraded over the years. This is both good and bad, since we now own better vacuum outlets but they are not all the same.
The oldest style is pictured below. It is turned counterclockwise to open to vacuum and clockwise to close. Unfortunately, there is a tiny pin inside which can get off track, making it impossible to open or close the outlet and causing the outlet to leak. It gets off-track if you overturn it, so please never force these outlets.
The second-oldest style is similar to the one above, but it is grey in color. It is less susceptible to damage if you turn it too hard.
The newest style has a lever to by-pass the over-turning problem. It is turned on by moving the lever from the closed position (horizontal) to the open position (vertical). The outlet itself should always be in the full-on (counterclockwise) position - you should never have to turn the the valve itself. The two pictures below show the off and on positions of the lever.
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