Underneath the front brake lever is a little microswitch that often goes bad. Pull the wires off the little switch and touch them together while the motorcycle is running (you won't get shocked). If you light the brake light, you've proved your switch is bad.
I had to replace my front brake light switch, the tip of the plunger that the lever releases when you squeeze the brakes had broken off. If you have the round cylindrical style switch, you can get them on Ebay or from David Silver Spares. Those tend to be slightly harder to find than the flat square shaped switch.