2002 F55 STS, 2014 Explorer XLT, F-150
The brake master cylinder cap IS airtight. If you look closely you'll see the cap gasket is actually a rubber bellows that flexes in and out with fluid use. The metal cap itself has a tiny breather hole to allow air to the outer side of the bellows. I'll bet some members have removed the cap, seen the bellows flexed downward and wondered why ........... it's designed that way. You simply check/top off fluid level and push the bellows back into place.Never heard something like this. First the reservoir of the master zylinder is not airtight, because temperature, wear of the brake pads which forces the pistons in the calliper to move out, which leads into more needed brake fluid in the system and therefor less fluid in the master zylinder, leads into change of fluid quantity in the master zylinder. If the master zylinder would be airtight the system would not work by design. To much fluid would simply come out at expansion. That could not cause this problem. Lower grade brake fluid in the system? How likely is that? More likely is, that the fluid is old and full of water...
Why the sealed bellows? Because brake fluid is hygroscopic - it LOVES water and will absorb it from the humidity in the atmosphere. Brake fluid should - should - be exchanged once every 5 or 6 years.