Brake fluid

Brake fluid transmits force from the lever or pedal hydraulically to the caliper pistons. Motorcycles mostly use glycol-based DOT 4 or DOT 5.1; the correct type is marked on the master cylinder cap.

Glycol-based fluid is hygroscopic – it absorbs moisture from the air and its boiling point drops. Under hard braking the water can boil, creating vapour bubbles: the lever goes soft and the brakes fade. That is why brake fluid is normally changed every 2 years or per the manufacturer's schedule; dark fluid means the change is overdue.

Never mix silicone-based DOT 5 into a DOT 4/5.1 system. Brake fluid attacks paint and plastic – cover the area and wipe up drips at once. Bleed the system carefully after changing the fluid.