Motorcycle Repair: Brake light does not work, generic question, light bracket

Question
How to I troubleshoot my brake light not working when brakes are applied.  I check and replaced the bulb.  Whenever the tail light is on both element inside the bulb are lit.  Everything else works. (Turn signals running light)

Answer
Harry, generally I prefer to know what you are working on, but this is a somewhat generic question and a real puzzler for a lot of people.

There is a high probability that your taillight ground path is open... not grounding. When you turn on the tail light or actuate the brake light, the voltage comes out to the filaments, but has no place to complete the circuit. Ya gotta have a + and a - in each circuit in order for it to function properly.

In your case, the power tried to find a lower potential return path and the only possibility is to return back onto the closed adjacent wire or electrical path... in this case, the other bulb filament.

Depending on whether you have a plastic fender or light bracket, you will have to determine and secure the proper grounding for your light assy.

Bill Silver