Well my brake switch was broken. The rear brake pedal would activate the brake light but the front brake lever wouldn’t. I checked on the brake light switch and found it wasn’t working and was quite warm. With some patience and a couple of small flat head screwdrivers along the seam I was able to open the switch. It’s fairly simple as far as switches go, it’s normally closed and with the brake lever in its normal position it depresses the plunger which opens the contacts and breaks the circuit. When you pull in the brake lever it releases the plunger which brings the two contacts together and completes the circuit. In my case the contacts (red arrow above) were dirty and adding resistance to the circuit.
I removed the switch mechanism from the plastic housing and cleaned it with electric degreaser, sandpaper, and a toothbrush. Be careful not to lose the tiny plastic plunger when you remove it.
Once everything was cleaned and shiny I replaced the switch mechanism and plunger, added a dollop of dielectric grease, then snapped the housing closed again. Plugged it back into the bike and voilà, brake lights are no longer broken.