If the light is even flickering at all, you're down on fluid. Best bet is to flush the system (it's probably due - every 2 yers) and use new fluid, although you can add any DOT3 brake fluid (IIRC, check your owner's manual to be sure of the spec) temporarily. The Ford Heavy Duty DOT3 fluid is actually one of the better brake fluids available BTW, though I'm partial to Castrol LMA myself.
The bigger question is why you are down on brake fluid. It could be because of normal pad wear causing the calipers to be extended more, thus holding more fluid, but I've never had that actually occur on any of my cars. You should find out why it's low and repair any problems ASAP.
Scott
