Most vehicles today are equipped with a brake fluid level warning light. The warning indicator is linked to a float and electrical switch in the master cylinder reservoir. When the brake fluid falls below a specified level, the float will trigger the electrical switch and a warning light will illuminate (in some cases, a message is displayed on the dashboard). The warning indicator will come on while the brake system still has enough fluid to properly function. This way, it acts as a precursor to the brake hydraulic system red warning light, which indicates a potentially catastrophic failure of the brake hydraulic system.
The Bulb Check: Brake Fluid Level Warning Light
When you start the vehicle, the brake fluid level warning light will illuminate, but it should go off after 1 to 2 seconds. If it does not, there is a problem with either the brake fluid level or the brake fluid level warning light system.
If the vehicle is equipped with a messaging system, no message will display as long as the brake fluid level is at an acceptable level. The message system is basically a "no news is good news" type of system. When a brake fluid level message is displayed, it means there is a problem with either the brake fluid level or the brake fluid level warning system.
What to Do: Brake Fluid Level Warning Light
If the brake fluid level warning light illuminates or a brake fluid level message is displayed, the red brake hydraulic light is not displayed, and the brake system performance feels normal, then take the vehicle to a qualified repair shop for a complete brake system inspection and diagnosis as soon as possible. The brake fluid level warning light can be an indicator of wear in the brake pads and/or shoes. As the brake linings wear, the brake caliper pistons will be more extended, causing an apparently low fluid level.