I've got a 87 Ford L9000 and the brake lights are staying on continuesly?

I've replaced the brake light switch and nothing changed, if I disconnect one of the wires for the brake switch the lights with go out, any ideas on where to start next

Either you brake system could be low or your parking brake has an issue.

Sounds more like the switch is out of adjustment

Usually when you disconnect the brake light switch it is either bad or just not adjusted properly. If the switch is not adjusted right the brake lights will stay on all the time. Try pushing the switch plunger in by hand and have someone look to see if the brake lights go out. If they go out adjust the switch so that the pedal contact pushes the switch plunger in farther.

Make sure your trailer hand brake lever is all the way up, and also spray some WD40 at the foot valve plunger, they sometimes stick.
The best way to test if it is a switch or a foot pedal or trailer valve is to drain all the air from the truck every last drop. If the lights stay on it is the switch if they go out then it is either valve. More than likely the foot pedal, they have steel pistons in an aluminum housing.

Usually the brake light switch on the pedal is broken off or the wire has become crossed as if you are always applying pressure to the pedal. ALSO, this can be a symptom of a master cylinder starting to fail or the power brake booster is stuck. If the pedal feels totally normal, then the brake pedal switch is likely.

HI so what about the real brakes should you not be attending to them first.