Updated: May 20, 2021
The central vacuum is a vital piece of office equipment and as touched on previously, your first line of defense in infection control so it behooves you to make your vacuum's job easier.
Whenever possible, you should have your vacuum working with gravity rather than against it. The main vacuum trunk lines should slope towards the central vacuum. The manufacturer's of vacuums all have specifications for how much slope is recommended to be certain you've got gravity working with you.
If it's unavoidable to go against gravity at some point, you always want to go above the main line and end with a downward bend so gravity still works in your favor at the terminal point. This also prevents the main line from feeding into the branches. The diagram is a commonly used overhead vacuum line configuration.
Gravity can be a harsh mistress, try not to fight against her. Whenever doing anything with your vacuum, always have things flow downhill if at all possible.