Topping off brake fluid seems simple, but there is a right level - and yes, you can overfill the reservoir. It is a less common mistake than running low, but it causes its own headaches. Here is why the fill line matters.
Fill to MAX, not to the top
The reservoir is marked with MIN and MAX lines, and the correct level is at or just below MAX - not filled to the brim. That gap above the fluid is intentional: brake fluid expands when it heats up, and the space lets it expand without being forced out. Fill past MAX and you remove that room.
What overfilling causes
An overfilled reservoir can overflow as the fluid heats and expands, and brake fluid is corrosive - it strips paint and damages anything it drips on under the hood. A too-full sealed reservoir can also build slight pressure that makes the brakes feel like they are dragging. Neither is catastrophic, but both are avoidable.
The pad-wear connection
Here is the subtle one: the fluid level naturally RISES as you push worn-out pistons back in with new pads. If you top a low reservoir to MAX while the pads are worn - see why is my reservoir low - then install fresh pads, the returning fluid can overflow. That is why you should check and sometimes remove a little fluid when doing a pad job.
Getting it right
Keep the level between MIN and MAX, use only the correct fluid type from a sealed container, and do not overfill. If it is low, find out why before topping off - see brake fluid flush guide. Pair correct fluid with quality pads and fresh hardware.