If its bloated than at some point somebody added power steering fluid to the master cylinder instead of brake fluid. This is very bad. You will likely need to replace the calipers, wheel cylinders, all the brake hoses, and the master cylinder. The entire brake system will need to be well bled in order to flush out the powers steering fluid otherwise you'll end up replaceing one or more of the above more than once.
Granted this will be necessary if the rubber seal on the cap is indeed bloated. Swing by a brake shop with the cap and ask them if it is if you are not sure what i'm talking about.
We usually see a couple of these a year. The price tags usually run into the thousands.