Someone already said it, both e-brake on and the proportioning valve seeing a pressure differential will trigger it.
Mine came on as my master cylinder failed. I wouldn't discount that light, make sure you aren't following folks too closely, you lose brakes sitting at a light you better be quick with the shifter, or get your ebrake hooked up.
Check first and foremost for leaks, barring anything visible, you need to ensure the electrical portion is working correctly.
If you pull the connector from the proportioning valve, and ground it, the light should come on. If it does, then you know the circuit is working right. I haven't dealt with the truck ebrake save when I installed the harness to the body, but IIRC, there is a single prong connector on the ebrake switch, and I've had those switches break in my car, which turned the light on as well.
Make sure the circuit is working right by testing the combo valve connector, make sure the ebrake switch isn't goofing up, and you should know whether something is really wrong with the brakes, or if there is a wiring issue.
FWIW, I figured at first something was up with my brakes as the pedal was a bit spongy all the time, but since the truck stopped fine and never caused any problems (until the light came on) I just assumed the spongy pedal was 'normal" for my truck.