Master Cylinder how bad does this look? w/ pics

[HO455]

Originally Posted by Steeveedee

The top seal sinks into the master reservoirs when the fluid disappears, right? Such as into the calipers to take of the wear volume of the pads.

That is correct. The seal has the accordion (?) pleats in it to do just that. The idea is to limit the fluid's exposure to air and the moisture it contains. The fluid will absorb moisture out of the air which leads to internal corrosion. (It is best to check your brake fluid on days with low humidity. Not that I ever remember that. You folks in the southwest once again have it better). When the fluid level drops the seal drops without pulling fresh air into contact with the fluid.
If you just put an air tight lid when the fluid dropped it would pull a vacuum and nature being abhorrent would end up pulling air into the reservoirs through the end of the master cylinder. Don't ask how I know that.