"Low" is the same thing as grounded - either a faulty neutral safety switch or a faulty parking brake switch. If the "brake" light is not showing on your dash, it's most likely the neutral safety switch. Take your car somewhere you won't run into anything and try to start it in drive or reverse. This will tell you for sure if it is shorted out. If it was not working at all, the car wouldn't start in park or neutral.