In a '68, there's a switch in the distribution block that closes if you lose pressure in either your front or rear systems. This makes the light come on hopefully telling you to check your system for failure. I bet '69's re the same. If you find something wrong (blown cylinder, broken line, leaky fitting, etc.), then the light did its job.
Fix the problem and re-center the warning light valve in the distribution block. That's where the fun starts. I have the instructions somewhere. If I can find them, I'll post them.
The brake function is unaffected by an "un-centered" switch valve. If you don't want to mess with re-centering it, just disconnect the switch and the light will go out. Ran mine for years that way because I didn't know how to re-center the valve.