Food for thought: Exact same experience on our '78 not too long ago. Couldn't find the location of loss of fluid, thought it might be the master cylinder, or booster going bad w fluid migration into bellows, etc.
In our case - Same loss of rear fluid turned out to be rear brake cylinders that could only be found & seen when pulling rear wheels and brake drums...the 2nd time.
1st time removal nothing seemed wrong...or leaking
