There are a ton of threads out about the brake warning indicator light being on, most of which is solved by just adding brake fluid or replacing your brakes. I had the brake warning light come on after wheeling at Jeep Beach and no other symptoms of any brake problem (leaks/worn pads/etc), so here is what I found.
Bottom line…if you’re running an air compressor and the bracket mounts to the master cylinder (as many do), make sure the wires from the master cylinder reservoir sensor are not pinched between metal.
The wires on the master cylinder reservoir sensor (plugs in pass side of res) were jammed between the master cylinder and the ARB compressor bracket which bolts onto the front end of the master cylinder housing. Shotty install on my part (rushed, dark, been drinking). While wheeling last weekend, these wires had gotten crushed from getting bounced around, resulting in a short which caused the warning indicator. Pulled the compressor, re-ran the sensor wires away from the brackets and wrapped what was left of the wire’s coating in e-tape. Reconnected, no warning indicator. Reinstalled compressor/bracket, made sure the sensor wires were not affected, no warning indicator.
If you’ve got the brake warning light indicator and can’t find the problem AND are running a master cylinder mounted compressor…one more area to check!