Keeping an eye on brake fluid is important for a number of reasons. Low fluid levels may indicate that the brake pads or shoes need to be replaced soon, but this can also be a sign that there is a leak somewhere in the brake system. Consult the owner's manual to help you locate the brake fluid reservoir. If it is at or near the 'full' line indicator, you should make a note of the date you inspected it and check back again regularly to keep an eye on things. Dark coloration of the fluid is a good indicator that it needs to be changed.
The first thing you can check when the brakes are acting up is the brake pads, which are often visible through the wheels. If the pads are thin, chances are that this could be the cause of the low fluid levels. However, if the pad thickness is okay and the brake shoes have recently been replaced, the brakes are likely acting up due to a leak. The system is complicated and there are multiple points where leaks may be occurring.
When a vehicle is leaking brake fluid, the brake pressure will be reduced, and in some cases even completely lost, which is extremely dangerous. If it is not corrected as soon as possible, a leak could render a vehicle undriveable or cause an accident if the brakes give out entirely while the car is being driven.
One way to determine if there is a leak is to pay attention to the feel of the brake pedal when you are driving. If the pedal feels soft or doesn't offer any resistance and just drops to the floor of the car when you brake, try to pump the brake a few times to build up pressure. If it doesn't work, chances are that there is a serious leak somewhere in the brake system.
When pressure does build up after a few pumps, then there may be air in the brake lines. Bleeding the lines will work to clear the air from older cars, but this may not work for newer vehicles.
In order to determine where the leak might be located, the easiest method is to park the vehicle on a level surface and let it sit for a few hours. Wherever the leaking fluid falls on the ground below the car will narrow down the possible leak points.