As long as there are no leaks at the hose connections and all air was removed from the hose at installation, no air can get into the system. The fluid is always going to be at the bottom of the reservoir (assuming you maintain the fluid level) so only fluid can be drawn into the hose and master.
my rear brake the needs bled every 3 months despite being constantly sealed says otherwise.
if this were the case, brake bleeding wouldn't exist.
air finds it's way in, no matter what.