Dude... this is incredibly simple... most likely. It's a dual filament bulb. That means there are two peices of metal that can light up inside the bulb. And when your lights flash/are on, the smaller one is lit up. But when you hit your brakes, the larger one lights up. The problem is that the larger one is burnt out/broken, but the smaller one is still in tact, which is why it still works, sometimes. Just get a new bulb.