Brake light on dash comes on at 40 mph

My brake light on my 91 Sierra comes on as soon as I reach 40 mph. If I keep the truck below that speed the light never comes on. I have checked for codes and there are none. I tested the resistance on the speed sensor and it is within specs. I was told the speed sensor also conects to the speedo which works fine. I replaced the EBCM but the light still comes on. Has anyone experienced this prroblem? Any help would be appreciated.

Mariano

I finally figured out why the light was coming on at 40 mph. It ended up being a faulty brake light switch. I replaced the switch and the light no longer comes on. I also noticed that my cruise control is also working now. Hope this helps anybody with the same type of problem.