I am in the process of changing my Brake Master Cylinder. But what I have noticed about 2 - 3 months ago while filling the brakes, is that the brake cover gasket was deformed whereas it was sucked down into the actual master cylinder? Really weird looking and I could not straighted it out at the time. Parts store doesn't have any that big so ended up buying a new cover. Now, looking at it again last night while bleeding my brakes, this NEW gasket is once again deformed as being sucked into the master cylinder. I have had a Brake Light on for some time now but as I have looked into that, my brakes have been working fine. (light reason could be a couple of things among them is electrical). In the process of replacing most brake related parts.

But why the deformity? My Brake stands to spin my tires taking it's toll on my Master Cylinder?

 

The only time I'm ever seen the rubber components deform is when a NO-COMPATIBLE fluid has been added to the system which could also account for a brake light staying on. If someone has added a contaminated can of brake fluid, the rubber seals in the system swell and deform because they're not compatible with the contaminated fluid. Oil based petrolium products (oil,power steering fluid etc.) will do just that.
I would suggest draining all the fluid in all the brake components and refilling the system, bleed it and repeat again to flush as much out as possible.
You could take a sample out and have it anylized. We have our fleet vehicles oil sampled and tested at every oil change.
It's done through a company called "WEAR CHECK"

 

Thanks for that info. I am planning to do a complete brake drain since I am going to replace my Master Cylinder.