Originally Posted by
cpach
It should be fine. If it feels good, it is good.
If you deceased ambient pressure on a hydraulic brake with significant air in it it would probably just expand the bladder in the resevoir slightly, slightly extend the pistons, or leak out of the reservoir. None of which are big deals.
Also the number of above posts that do not understand that there is a reservoir in a hydraulic disc brake... The system is only really sealed when the brake lever is depressed because the piston in the master cylinder moves past the opening to the resevoir. The reservoir is necessary because the pistons advance with pad wear and the additional fluid needs to go somewhere. Also the resevoir is usually the highest point in the system so air tends to collect there so it can do no harm.
The reservoir is also sealed to ambient air pressure. What you are describing has nothing to do with changes in air pressure. If the bike is upside down, at just the right angle, for long enough, and the lever is pulled, there is a slight possibility that air could get into the line. Chances are though, there will be no one in the cargo hold to compress the brake lever.