Your mechanic needs to get out more. Hydroboost is a weak point for all vehicles that use it, particularly GM and Ford trucks. Search for Hydroboost problems and you'll see plenty of truck forum discussions about it.
It typically happens near the end of service life for us... maybe 75k and 7-8000 hours.
The leak tells you it's going to fail. When it fails the brake pedal will suddenly go almost all the way to the floor. It will still stop, but the brake travel and lack of assist is scary.
Don't try to rebuild it, and spend the extra cash for a new unit. Rebuilds won't leak, but they'll often growl like a common power steering unit that's going out.