Braking systems are not closed to the atmosphere.
The reservoir cap is vented, and rubber hoses are somewhat permeable.
Some folks do advocate opening the bleeders when the caliper pistons are retracted for a pad change. The idea is that this prevents the dirty fluid in the calipers from being forced back through the master cylinder. The fluid within a calipers tends to be the most contaminated, since they are at the lowest point in the system.
1986 300E 5-Speed 240k mi.