Most reservoirs are translucent, so checking the level is easy and should be done at least once a month. If the level is slightly low, top it off by pouring fluid into the reservoir. Brake fluid needing to be topped off frequently may indicate that there is a leak in the system, possibly in one of the brake lines. Also, avoid spilling brake fluid on painted surfaces, since it will damage automotive paint. It's a good idea occasionally to remove the lid and get a close look at the fluid.
Fresh fluid is clear or translucent.
If the brake fluid appears dark, it's time to have it replaced. With the Car on a Lift Check the brake lines underneath the vehicle. The metal lines run the length of the vehicle, so check them all along their span. Make sure that they are flexible, that there are no cracks, kinks or leaks, and that they are clear of any moving parts or heat-producing elements such as the exhaust system. Check the parking-brake cable to make certain that it is clean, undamaged and able to operate properly.