well based on the fact that he bleed the system and checked all four corners it certainly does seem that the master cylinder is a good bet. Especially considering the very classic systems of a blown master cylinder.

Do check the rubber lines going to the caliper though. It's pretty likely that they are shot as well. If they are over ten years it's time to replace them and if the system is going to be apart now is really the time.

P.S. In humble opinion only