When you push on the brake pedal, you are pumping brake fluid through the system to the brakes. For most cars, the brake-fluid reservoir is translucent material, so you can easily see if it's full. And like most other systems, today's brake systems are well sealed and almost never leak. If the brake reservoir is full your brake system has integrity. If fluid is low you need to purchase brake fluid and refill the reservoir. Check it weekly. If the fluid levels are down, you must fix it immediately. You could have a problem that can make driving dangerous. Like coolant, brake fluid gets old and must be replaced at long-term intervals. This should be done by a shop.
Ready to pick up the wrench yourself?