2004 Official factory service manual for Yamaha YFZ450S ATV Quad., Page 107Get this manual
ADJUSTING THE REAR BRAKE LIGHT SWITCH CHECKING THE BRAKE HOSES
ADJUSTING THE REAR BRAKE LIGHT SWITCH NOTE: The rear brake light switch is operated by movement of the brake pedal The rear brake light switch is properly adjusted when the brake light comes on just before the braking effect starts1Check: rear brake light operation timing Incorrect Adjust
2Adjust: rear brake light operation timing