[Mike_The_Grad]

That's correct about the diaphragm. It's easy to eliminate an external master cylinder leak, just wipe the bottom with a clean cloth, should be bone dry, unscrew the two bolts that connect it to the booster and slide it forward an inch or two, should be bone dry. Don't fool yourself into thinking it's "moisture" or "condensation" that stuff will evaporate. Brake fluid will not.