Every car on the road for the last several decades has come equipped with some kind of brake indicator light in the instrument cluster. This light typically says something like “brake” illuminated in red. On some international models the light may just be some kind of symbol to represent the brakes in some way. Sometimes it might be both.
The brake warning light should not be confused with the ABS malfunction indicator lamp. This light usually says “anti-lock,” or “ABS,” or something like that. This indicates a fault in the ABS system which is actually separate from the regular service brakes. Some cars are also equipped with a warning light that indicates when a brake light bulb is burned out. This is obviously a separate system as well.
The brake warning light can be deceptive because it never means what most people think it means. Many people think that when this light comes on it means the brake pads need to be replaced because they are worn out. In some scenarios the illumination of this light might be related to worn out pads but the relationship between the light and brake pad wear is indirect.
The brake indicator light will illuminate for a few different reasons. The first and perhaps simplest explanation is the parking brake being applied. When the parking brake is applied there is a small switch usually attached to the lever, or linkages that will close when the parking brake lever is pushed or pulled. When this switch closes it completes the circuit that supplies power to the light and it comes on. So if you are driving with the brake light illuminated the first thing to do is check to make sure the park brake is not partially applied.
The next most common thing that causes the brake light to come on is low brake fluid in the master cylinder reservoir. This low fluid is usually the result of brake pads wearing thin. As brake pads in a disc brake system wear out, fluid moves from the reservoir above the master cylinder into a space behind the caliper piston. The piston slowly moves out of the housing as the pads wear so that the friction material in the pad will always be close to the rotor. This ensures a prompt response when the brake pedal is pushed.
If the fluid in the reservoir is low it’s important the brake pads be inspected for wear. Never top off a brake fluid reservoir just to turn a light off. Upon inspection, a technician might conclude that your pads are fine. At this point it would probably be okay to add a little fluid since it seems the reservoir might not have been topped off properly the last time the brakes were serviced.
A leak in the brake hydraulic system can also cause the fluid in the reservoir to be too low. This is another reason to have the brake system inspected when low fluid is found. A trained technician can easily spot a brake fluid leak and recommend repairs. Most of the time when a fluid leak is bad enough to cause the level in the master cylinder to drop, the driver will probably notice a problem with brake pedal feel. If fluid is coming out of the system somewhere, chances are air is getting into the system. This will cause the brake pedal to feel spongy.
This leads us to the third most common thing that can cause the brake light to illuminate; A pressure imbalance in the braking system. Hydraulic braking systems are comprised of two separate circuits. Each circuit is responsible for creating, and sending hydraulic pressure to two of the four wheels. A pressure switch is mounted between the two circuits to make sure pressure is equal between them. If one circuit develops a leak it will not build proper pressure and will cause the brake light to come on. This is a very serious thing since it drastically affects the ability of the vehicle to stop safely. Nothing is worse than the feeling of a brake pedal that sinks to the floor.
A few other things can cause the brake light to illuminate. The light may come on when there is a problem with some other system and it may come on with other lights that are actually the ones indicating the problem. If the ABS light comes on, it indicates a problem with the anti-lock braking system. Quite often the brake light will come on at the same time. Also, on many cars when an alternator goes bad it will not only cause the battery light to come on, but sometimes other dash lights like the brake light will illuminate with it.
Like all the other malfunction indicator lamps, the brake light is important. Knowing why it comes on and what all the possibilities are for potential problems will help any driver stay in touch with their vehicle. Knowing how your car works, but also how it can go bad, makes the driving experience a little better. Don’t be deceived.