Brake light on dash always on.

I have a 91 1500 stepside 4x4 350cid. The brake light on the dash comes on every time I drive it. It seems to happen at around 30 mph but I do not understand this because it doesn't have abs. If I apply the parking brake the light gets brighter so I know it is not that switch. The fluid is full. Does anyone have any ideas?

Bad ground behind dash. The electricity is finding a ground through the brake warning lamp causing it to glow. You seem to have a wiring problem if it comes on at around 30mph.

I have been watching it more closely. It comes on at exactly 37mph. I read another post with the same problem at the same speed and someone said it was an ABS problem, but I do not have ABS. There is a black box mounted to the master cylnder, but there are no wheele sensors anywhere on the truck. On sunday I went through the entire truck and cleaned all body grounds. Oh and my cruise doesn't work now either.

What about rear wheel ABS? Could be the Vehicle Speed Sensor mounted in the tail shaft of the transmission.

Is that where it is? I looked at the wheels and saw no sensors. Wow don't I feel stupid. Thanks for the info I will check it out.