My running lights work but by break light wont

Check the bulb first. There are two filaments in the bulb, one for the brake light and one for the tail light. If the bulb is good, there are two switches that turn the brake light on. One is on the front brake lever and the other is a pressure switch in the metal brake line from the rear master cylinder. Work with the rear brake light switch first.

Find and follow the metal brake line coming from the rear brake master cylinder running back towards the rear brake. Underneath the bike's transmission, you'll find a tee fitting with a pressure switch in it. With the ignition switch turned on, one of the wires that are plugged onto the switch should have battery voltage on it. When you depress the rear brake pedal, both wires should have voltage on them. If not, replace the switch.

Once you get the rear brake light switch working, if the front brake light switch does not work, it must be replaced. To replace that switch, remove the two bolts that hold the right handlebar switch housing halves together. Loosen the throttle cable and idle cable adjustments. Lift the top switch housing half and remove the throttle sleeve. The front brake light switch is the small switch right in the front of the housing half. You must desolder the wires, replace the switch with a new one, and resolder the original wires to the new switch. Reassemble the throttle sleeve and switch housing halves. Adjust the throttle cable and idle cable. Check throttle for proper operation.

Good Luck
Steve