Instructions on the can of brake fluid usually say " keep tightly closed " between using. This is because brake fluid draws and keeps moisture... which at some point of percentage can cause steam in your brake lines... which is compressible and thus defeats the purpose of having a hydraulic connection to your brakes... or accumlates at points and rusts things in your system..

This of course is the reason regular flushing of your brake system is recommended..

However, with these brake bleeding mechanisms I am concerned that they are not good ( violate the rule on the can ) for keeping moisture from your brake fluid.

Perhaps either some dry gas ( like Argon ) or some bladder could be added to these units to keep moisture laden gas from the surface ( and under pressure at that ) of the brake fluid.