Originally Posted By: mechtech2
What's wrong with topping off brake fluid?
Modern master cylinders have seals that keep dirt and moisture out of the system. Every time you take the cap off you allow moisture from the air into the system and potentially allow foreign matter into the system.
Originally Posted By: mechtech2
You get a fresh shot of clean fluid to mix in with the old, and less air and concurant moisture in the reservoir.
If the pads are low or there is a leak, that is another story, but I'd still keep the reservoir filled.
The rubber baffle in the cap should not allow in any air. Clear fluid reservoirs are you friend. Don't open the cap unless needed.