when i serviced and replaced my entire brake system a small leak was found going from the master into the booster at the rear seals...the one real problem for rust and rot is on top of the rear sub frame as water vapor lives here....some will say it can be traced to the flex lines, as they get clogged with brake fluid cholesterol....the oem master has a weep hole in the rear lower section to indicate a leak, an after market on may not have this weep feature...
hope this helps
jake
hope this helps
jake