When brake fluid gets old and dark, the fix is not to add fresh fluid on top - it is to drain the old fluid out and replace it. Topping off does not solve the underlying problem. Here is why draining old brake fluid matters.
Old fluid is degraded throughout
Brake fluid absorbs moisture over years - see why brake fluid absorbs water - and picks up rust particles and rubber debris; see contaminated brake fluid. That degradation is spread through ALL the fluid in the system. Adding a little fresh fluid to a reservoir full of old fluid barely changes the overall condition.
Topping off does not fix boiling point
The main danger of old fluid is a lowered boiling point from absorbed moisture - see brake fluid boiling point. Since the moisture is throughout the system, only replacing essentially all the fluid restores a high boiling point. A top-off leaves most of the wet fluid in place.
Moisture corrodes from inside
Old, wet fluid also corrodes the ABS unit, calipers, and lines from the inside - expensive parts. Draining and replacing the fluid removes that moisture; topping off lets the corrosion continue. This is why makers set a time-based flush interval - see how often to change brake fluid.
The right approach
Do a full flush - push fresh fluid through until clean fluid runs from every bleeder, replacing essentially all the old fluid; see brake fluid flush guide. Top off only to correct a level, not as a substitute for a flush. Pair fresh fluid with quality pads and hardware.