Recently noticed the brake warning light on the dash of my 1977 GMC C15 did not light up when engaging key in on position. After reading other threads, I was able to activate the light by removing the wire from the proportioning switch and grounding it, which if I understand it right, indicates that the circuit is good. I then reattached the wire to the proportioning switch but the dash brake warning light did not light up. Does the proportioning valve switch ever go bad and is there a way to check it? I’d appreciate any input on what might be the issue. Thanks in advance.