Brake fluid has an affinity for water and takes on atmospheric water. You may think that the brake hydraulic system is sealed, however there is no such thing as perfect seals. Minute as it may be, some atmosphere enters the system and water comes in with it.
As brake fluid takes on water, it degrades. At lower levels, the brake fluid absorbes the water and holds it in suspension. The only damage here is that the brake fluid boiling point is lowered. Some brake fluid containers have "wet" and "dry" boiling points on it. The "dry" readings is fresh brake fluid with no water yet absorbed, and the "wet" readings is when the fluid has absorbed it's full capacity of water that it can still hold in suspension. An example may be something like 550�° F dry and 284�° F wet, so you can see that the difference is considerable.
If the brake fluid boils during hard sustaind braking, you will loose brake pedal since fluid is compressable and air essentually is not.
The next critical thing about old fluid is that as it degrades it will become gunky so that you have debris in the fluid leaving deposits. This gunk also becomes corrosive and starts to damage both rubber and metal internal parts. Also the gunky debris can cause havic with the ABS hydraulic control unit.
Changing the fluid is important then for many reasons.
