I've once had rising fluid levels in another much older car.
I was carrying a heavy load, and the handbrake cables pulled a bit even in their released position, slightly engaging the shoes. The consequence was the rear brakes got quite hot, the fluid vaporised in the pipes and displaced fluid backwards towards the MC, raising the level in the reservoir.
The brakes became spongy, but returned when it cooled down. Taking the load out restored everything to normal.
So I would say yes, something is causing the brakes to rub very slightly. The most likely culprit sounds like the servo being too tight so the MC push rod cannot fully retract.
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