Why is it that the brake fluid on bikes has to be changed every couple of years? No one I know ever changes the brake fluid in their cages, and no owners manual every says it needs changing. What's different with a bike?

David

 

As brake fluid ages, it absorbs moisture. The larger the moisture content, the less efficient the brake fluid is, and its boiling point is lowered, allowing brake-fade to happen at lower temperatures. No to mention is does get dirty, though the blackish tint is usually from the graphite lubricant that is during assembly of the master cylinder.

Flushing brake fluid is also recommended in cars, though it's often overlooked.

 

+1---What Redbeard719 said