Do what dub says! My brake light came on a couple of weeks ago and the first thing that I did was look at my reservoir under the hood and, sure enough, it was pretty low. I just checked it this morning while I was doing the oil top off routine while getting fuel (I almost said "getting gas" but I knew you people would have a field day with that ) and it seems to be low again. I must have a slow leak somewhere that I'm going to have to chase down.
"The major difference between a thing that might go wrong and a thing that cannot possibly go wrong is that when a thing that cannot possibly go wrong goes wrong, it usually turns out to be impossible to get at or repair."
-Douglas Adams, Mostly Harmless