There are a total of four rubber seals that help to keep brake fluid inside of your reservoir during use. When the rubber ages, it can crack, and harden leading to the potential for leaks.

The first pair of seals cover the brake fluid sensor floats. They can be easily seen on top of the reservoir. These are very visibly cracked.

The second set of seals, also called plugs, seal the area where the reservoir enters the master cylinder.

The reservoir is not under pressure when using the brakes so these seals can be in pretty rough shape and still work OK. However, if you bleed your brakes using a pressure bleeder you may find fluid shooting out! That will hint at a good time to change them...