brake troubles on 1990 Chevrolet 2500 Pickup

This problem is really on a 1985 Chevy 2500. I am having problems getting the pressure differential valve to reset to turn off the brake light on the dash. I changed out the master cylinder, then bled the brakes but I cannot get the light to go off. What is the trick to resetting this. I also still have a low pedal on the truck. The brakes seem to be working fine other than a low pedal and the light stating on all the time.

Asked by for the 1990 Chevrolet 2500 Pickup
Have you bled both the brake system AND the master cylinder? I feel as if you still have air in your system...causing both the low pedal and along with it the light.
I have bled both. I have used up a quart of fluid, working on this problem.
Other than bleeding the system, the replacement master cylinder you have may be faulty.

What problem did you have prior to doing repairs?
The brake light had comeon. The master cylinder that I replaced leaked all the fluid out of the rear reservior. It leaked out between the master cylinder and the brake booster.
Again, unfortunately, there isn't more to say, other than air may still be persistent in the lines, or the master cylinder could be faulty. Bleed the master cylinder first, then bleed the lines. I wish you success and luck!
The e brake when engaged can make the brake light come on! check the button and make sure the pedal is all the way up! you can stick a wire in the harness from each wire to short it and see real quick ??? I hope this may help if any at all! but it does sound like it is in the brake line
I've got it fixed now. It was the way I was bleeding the linesw that was keeping the light from going off.