Brake fluid is a type of hydraulic fluid used in hydraulic brakes in vehicles. It is responsible for transferring force into pressure and amplifying braking force. In other words, when you apply your foot to the brake pedal, brake fluid transfers this force into pressure to the front and rear brakes and stops the vehicle. It works because liquids are incompressible.
Therefore, brake fluid is a critical component of your vehicle. Anything that might degrade the quality of your fluid — such as moisture absorbed from the air — will hurt its performance and may ultimately hinder your vehicle’s braking.
For reliable and consistent brake system operation, the brake fluid must maintain a constant viscosity under a wide range of temperatures, including extreme hot and cold.

Check your brake fluid as often as you check the other fluids in your engine. A small decrease in the brake fluid is normal and can be topped off. However, if the level consistently drops, the cause should be investigated and repaired.