Most automotive professionals agree that glycol based brake fluids, (DOT 3 brake fluid, DOT 4 brake fluid, DOT 5.1 brake fluid) should be flushed, or changed, every 1-2 years. Many manufacturers also require periodic fluid changes to ensure reliability and safety. A properly functioning brake system is a key to a safe driving experience, especially if you drive a big rig, like a semi truck. So, if you notice any signs of a problem with your brake system, don't ignore them and have your brakes inspected as soon as possible.