Master cylinder and cover / breathing (vents)

[Leo B]

Please explain. How master cylinder and ventilation works in its cover?

Edit: I understand that when the fluid level changes during use, there must be breathing in the fluid space of the master cylinder. As I understand it, that's why there are also holes (vents) in the gasket. How about the cover?

Edited Thursday at 02:25 PM by Leo B

[Leo B]

I came to the conclusion that the lid/cover should not breathe. This way the liquid does not absorb moisture. Under and overpressure remain between the seal and the lid. A very small amount of air may leak between the cover and the screw. 

[Mark L]

Most newer master cylinder covers have a gasket with a built-in diaphragm or bellows. As the brake linings wear and fluid is pushed out to the cylinders, the diaphragm extends into the ullage space above the top of the fluid. This keeps a somewhat constant air volume and pressure above the fluid.