96 f350 brake light at idle

[mcgraw]

I have a 96 ford f350 4x4 7.3l I need to get this truck inspected asap and just recently the brake lt comes on when the truck idles for mor than 5 or 10 sec, as soon as you throttle up the light goes out. Brakes function fine, air pump on motor not old, and vaccume line on booster has good vaccume. Any Ideas????

[nitrogen]

are we talking the brake warning light on the dash? if so something is changing when you add revs, could be low fluid but i don't think 96 had a low level warning, possibly loose wire to brake valve vibrating and grounding. most likely though is that it is losing vacuum somewhere and it can't quite keep up at idle. put a vacuum gauge on the brake booster and monitor it at idle and revved up.