Sounds like your system needs some parts replacement, but it won't be cheap. The brake hydraulic accumulators on the TEVES braking system tends to wear out over time. The accumulator was designed to hold pressure for several braking applications. There is a separate electric pump that brings the pressure back up. The system ws designed to hold pressure for 3-5 brake applications, then the pump is supposed to come on and bring the pressure back up. The brake light is probably coming on because your system doesn't have enough pressure anymore, the accumulator leaks to much. If you hear the pump coming on more than every 3-5 brakings, this could be the case.
What you need to do is replace the accumulator, pressure switch, and relay. Parts alone, thru someplace like 5 Star Ford or Ford Parts Network will probably cost you close to $500 or so if I remember right, I had mine done about 1 year ago. The job can be a pain in the butt, so I supplied the parts and had a local shop do it for about $100. You probably want to do this ASAP if your brake light is on or if you hear the pump every braking, or worse if you don't even hear it. If the pump goes bad, you have no brakes. Oh, and the cost to replace the elecrtic pump is close to $1000 alone. Replacing the accumulator will help to save your pump by cutting the duty cycle down to about 20%-30%, thus extending it's life as originally intended in the design.
Hope this helps. Here's a link to the old site where I originally asked about and the directions if you want to do the job yourself:
http://www.lodonline.org/dcforum/DCForumID20/10.html
Bill
98 Mark VIII LSC - Silver - 59K mi
86 Mark VII LSC - Black - 242K mi
More Info In Profile - Member Since Dec 2000