Brake Fluid Brake fluid is very important for safe braking, just like your brake pads. It is a hydraulic fluid that is also used for a hydraulically operated clutch. Over time, the fluid absorbs water, which increases the danger of vapour bubbles and acute brake failure. For this reason, brake fluid should be changed every 2 to 3 years. Tip: The Louis Bike Database shows you at a glance which brake fluid is suitable for your machine.