ABS and Brake Lights On

[internic]

my brakes work just fine but the amber abs light and red brake light will both go on while driving. they are not on all the time, about once a week they will go on while im driving and stay on until i turn the ignition off. any ideas? my fluid is pretty dirty and i need to change it out, but i dont know if that would be a problem. maybe a wheel speed sensor or something?