there are sensor inside the brake resevoir cap and the sensor some time get contaminated with brake fluids.just change out the cap with new float. that should fix the problem...

 

Make sure you bleed your brake lines. Did you just do brake work?

 

this prob comes by me alot, all it is usually is , go under the dash in your car, take a look at the little brake sensor that touches your petal arm usually it gets stuck, its like a 10$ part at most, just replaced mine and thats all it was