You CAN hang bikes vertically. My Stumpy gets hung up everyday by the rear tire and I have yet to have a problem with my Avids.
Maybe with new floor bikes..BUT as the pads wear the fluid level in the master cylinder's reservoir lowers.
With the reservoir then containing say 1/2 air and 1/2 fluid you can have issue.
In theory the rubber bladder that covers the reservoir should pull in as the fluid level drops. Unfortunately this usually isn't a 100% seal and some air is under the bladder..
Just sitting vertically is OK in truth but the issue comes along when you pull the lever. For some reason it is impossible for friends or visitors not to pull a lever as they walk by.
This seems to be the worst with Avids. It probably has much to do with the location of the port in the reservoir.
Try holding a used bike with Avids vertically and pull the lever a few times. In most cases the lever feel will go soft. With Avids you can return the bike to level and pump the lever a dozen or so times and work the bubbles back to the reservoir.
An Avid tech showed me this trick. Try it next time before you decide to bleed them.