Brake warning idiot light

I have a 1993 Toyota Camry with a 4-cylinder and 190,000 miles.
I replaced my master cylinder because it wouldn't hold a pedal. The new rebuilt cyl. Did not have a new resevoir so I flushed the old one with spray brake cleaner. Filled it with new fluid and bled the brakes by having my buddy pump the pedal. Brakes work fine but now the idiot lite is on. I think this only tells me that the fluid is low. It isn't. Could I have damaged the sensor on the resevoir by cleaning it? Or does the idiot light indicate a failure in the proportioning valve? Does it have to be reset?

Thanks