Boy has this thread gone all over the place!

Low brake pedal, inconsistent brake pedal, weird feeling brakes. Beware the load sensing proportioning valve! It lives on the frame over the rear axle. There is a bleed nipple on it that is often overlooked. It also sludges up and doesn't work right. Disassemble it and clean it. All by itself, this can frequently fix the whole problem.

Brake light on. The sensor in the fluid tank cover frequently requires the fluid to be at or above the max line in order to stay off. Top up your tank and this can fix things.

Brake light on, part II. If pushing the hand brake forward turns the light out, the red-neck fix is to tape the switch shut. The real problem is the cable not pulling the hand brake handle down. Clean up and lubricate the hand brake mechanisms under the truck.