Master cylinder failure, what could be causing undue pressure on the master cylinder and blowing the seals?

In 133,000 miles this vehicle has been through four master cylinders and two brake boosters. The first two replacement master's I believe were AC/Delco and now another genuine Toyota less than a month ago which is already failing. Twice there was fluid in the booster. Calipers are not sticking and there are no apparent leaks. I'm trying to help this owner, but the vehicle is in Florida, and I live in Wisconsin. Isn't there some sort of distribution valve that might be causing undue pressure on the master and blowing the seals? I know Toyota had a TSB out about using genuine Toyota brake fluid that contains certain lubricants. What are your thoughts on this issue? Obviously, her mechanic can't figure it out.