IIRC, the BRAKE light on the main dashboard cluster is not at all controlled by the brake pedal/brake switch. It is activated by one of two things only:

1. The Emergency/Parking lever is pulled/activated
2. The fluid level in the main reservoir is low and the float has tripped the sensor.

You can sometimes verify #2 by manually raising and lowering the sensor in the reservoir. If you have sufficient metal debris in the master cylinder, it can be enough to trip the magnet sensor mounted at the base of the reservoir.

If you have a dirty reservoir, I can only recommend a total drain, clean and flush of the current system. draining just the bowl, cleaning and refilling - you're likely to still get contaminants in the system later missed by not doing a total flush. Time consuming, but not the hardest job. Also gives you the chance toi remove the master cylinder and ensure it's not yet leakign into the brake booster.

Nothing more important on these big trucks than a good brake system!


<EDIT: Just re-read the 1st post, guess I should have paid more attention. X2 on the recommendation by FastEddy below. What I posted is a very common problem however and wouldnt hurt to check the Master Cyl at this point!)

Jim/MI-Mitsu