Let’s think about it a bit.....
If a bottle of brake fluid is kept with the cap on then it will have a VERY restricted change of air (if any). The cap may not be a perfect seal but, seriously, there is no airflow. The water content of the air trapped when screwing the cap on is what would be absorbed. Leaving the cap off is a different matter though. Far more exposure to moist air.
If you want to be pedantic then decant the left over fluid in a half full bottle into a smaller one so you minimise the trapped air volume!
Every two years I buy fresh fluid for the replacement and keep the left over in case of a top up, but dispose of it when doing the change....it isn’t going to break the bank, is it?
