Check all your brake lines and hoses for a tiny leak, even if it seems like non-existent seepage at this point. I think the brake switch in the master cyl. reads "rest" pressure and low fluid too. I had a rear flex hose that didn't really drop the fluid level in the reservoir that much, but weeped or lost just enough pressure to trip the Brake light a few days before the rubber or crimp went all the way.
I refilled the reservoir a few days before the hose went, but still saw the warning light flashing on and off, then on steady until I fixed the hose. Once the hose was replaced, no more Brake Light.