If you are saying that the gasket for the master cylinder cap is blown out, then that is a sure sign of contaminated brake fluid. Someone put the wrong fluid in it !
So now all the rubber components in the brake system have failed- master cylinder, hoses, calipers, rear wheel cylinders, etc. All these would have to be replaced, the rest of the system needs to be flushed, add new fluid and bleed system.
See this link for more info...http://www.cartruckinfo.com/brake-fluid ... ation.html