Volvo: Brake Failure warning light, brake failure, instrument cluster

Question
Hi Jason, I've got a 1989 240DL sedan. The other day the brake failure light remained on after start-up, after several miles it went out for the remainder of that trip. Since that trip the brake failure warning light has been remaining on non-stop after start-up. The brakes still work great and the pedal is firm etc, fluid level is perfect with no detectable leaks anywhere, pads have plenty of meat, electrical connection on differential warning valve clean and tight. Can you please suggest a course of action to troubleshoot this? Thankyou for whatever help you can give.
Regards, Don

Answer
My first inclination would be to check the differential warning sensor itself, it might have failed.  I can't remember if it completes a circuit or breaks a circuit to set off the light bulb, but disconnect the connection and short the circuit to see if you can get the light bulb to go off and on.  If you can, then the connection to the instrument cluster is ok, and I would look to replacement of the valve sensor; or pull one off a used car at a junk yard.  If you can't get the light to go on and off, then the blub is being lit from somewhere else and I would trace back from the instrument cluster looking for an electrical short.