If you want to keep that master cylinder for some reason, it can be cleaned, rebuilt and put back in service. Once removed, a diluted solution of muriatic acid will clean the rust right out. if the bore is rusted as well, it could be sleeved at a place like White Post Restoration if your that invested in keeping that MC. As far as why it is rusted in one chamber, most likely the rubber diaphragm under the cap had a crease, tear or pin hole that let air come into direct contact with the fluid and absorb the moisture from the air. The rear chamber was working properly. The air is supposed to enter the vent in the lid and push the diaphragm down slightly when braking or if the fluid level drops. Little to no outside air gets into the fluid area directly. That looks like a simple/common MC that could be replaced with a new one inexpensively.
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